What is code review? Process and types

Introduction

A developer submits a pull request that passes every automated test. During review, another engineer spots a missing authorization check that could let users modify records they don't own. The code works as expected under normal conditions, but an overlooked edge case creates a security risk. Code review provides an opportunity to catch problems like these before changes enter the shared codebase.

This guide explains how code reviews work, the different review approaches, what developers should examine, and how teams can make reviews more effective.

What is code review?

Code review is a software development practice where one or more developers examine source code written by another developer to identify potential issues, assess implementation quality, and provide feedback before changes are merged into a shared codebase.

During a review, developers examine whether the code meets its intended requirements, follows established coding standards, handles errors appropriately, and remains maintainable as the application evolves. Depending on the change, reviewers may also evaluate security, performance, and architectural decisions.

Most modern development teams conduct code reviews through pull requests or merge requests on platforms such as GitHub and GitLab. These workflows allow developers to inspect proposed changes, leave comments on specific lines, suggest improvements, and record approval decisions.

For example, when a developer introduces a new payment validation feature, another engineer might review how it handles invalid transactions, duplicate requests, and interactions with existing payment services.

The review creates an additional quality checkpoint while giving developers an opportunity to share technical knowledge and examine implementation decisions together.

Why is code review important in software development?

Code reviews help engineering teams evaluate changes before they become part of the main application. Beyond identifying implementation problems, regular reviews improve shared understanding of the codebase and support consistent engineering practices.

Here are six important benefits of code reviews in software development.

1. Identify bugs and defects early

Developers can overlook conditions outside the scenarios they considered during implementation. A second engineer may notice incorrect assumptions, unexpected inputs, or interactions with existing functionality.

For example, a calculation designed for positive values might produce incorrect results when processing zero, negative numbers, or missing inputs. Identifying such issues during review can reduce the debugging and corrective work required later.

2. Improve code quality and maintainability

Readable, well-structured code is easier for other developers to understand and modify. Reviewers can identify unnecessary dependencies, duplicated logic, confusing abstractions, and implementation choices that could complicate future development.

These improvements become particularly valuable in large codebases where several engineers work on interconnected components.

3. Strengthen application security

Reviewers can examine changes for insecure coding practices, missing authorization checks, unsafe input handling, and potential exposure of sensitive information.

Changes affecting authentication, permissions, payment processing, or personal data may require additional scrutiny from engineers familiar with the application's security requirements.

4. Maintain consistent coding standards

Engineering teams establish conventions for naming, error handling, architecture, and code organization. Reviews help ensure that new changes follow these shared expectations.

Consistency also makes unfamiliar parts of the codebase easier to navigate, particularly when multiple contributors work on the same project.

5. Encourage knowledge sharing and collaboration

Peer code review exposes developers to implementation approaches and system components outside their immediate responsibilities.

For instance, reviewing a change to an authentication service may help another developer understand how the application manages sessions or permissions. Over time, this shared knowledge reduces dependence on individual engineers for particular parts of the system.

6. Reduce technical debt and future rework

Some implementations satisfy immediate requirements while introducing complexity that becomes expensive to maintain.

A reviewer might notice that a new feature duplicates an existing service or creates an unnecessary dependency. Addressing the concern before merging can prevent additional maintenance work.

This makes reviews a useful practice for managing technical debt as a product and its codebase evolve.

What are the different types of code reviews?

Development teams use different review approaches depending on the complexity of the changes, collaboration preferences, and required level of oversight. Some involve direct interaction between developers, while others rely on asynchronous feedback or automated analysis.

The six common types of code reviews are explained below.

1. Formal code review

Formal reviews follow a predefined procedure with designated reviewers, documented findings, and established completion criteria.

They are useful when changes require detailed examination, documented approval, or adherence to specific organizational procedures.

2. Peer code review

Peer reviews involve developers evaluating code written by their colleagues. Reviewers assess the implementation, identify concerns, and suggest improvements based on their technical knowledge.

Teams commonly select reviewers who understand the affected component or programming language.

3. Pull request-based code review

Pull request reviews happen through a version control platform. Developers submit proposed changes, and reviewers examine code differences, discuss issues, and provide approval or revision requests.

This approach supports asynchronous collaboration because developers can review and respond to changes at different times.

4. Over-the-shoulder code review

An over-the-shoulder review involves one developer walking another through the implementation.

The reviewer can ask questions and provide feedback immediately, making this approach useful for focused changes or decisions that benefit from direct discussion.

5. Pair programming

Pair programming involves two developers working together on the same implementation, commonly with one writing code while the other considers design decisions and examines the changes.

Feedback occurs throughout development. Teams may still require a separate approval before merging the completed work.

6. Automated code review

Automated code review uses tools to examine source code for potential problems, such as rule violations, certain security weaknesses, and maintainability concerns.

Static analysis, linting, and AI-assisted review can support human reviewers by identifying issues that warrant further investigation.

Comparison of code review types

These approaches can complement one another. For example, a peer review may happen through a pull request, with automated checks running before the reviewer begins examining the implementation.

How does the code review process work?

The code review process usually begins when a developer prepares a change for evaluation and ends when the required feedback, validations, and approvals are complete. The exact sequence depends on the team's repository configuration and development practices.

A typical code review process involves seven steps.

Step 1. Prepare the code for review

The developer completes the implementation, performs a self-review, and runs relevant local tests.

Preparation also involves removing temporary debugging code, checking for unintended changes, and confirming that the implementation addresses the assigned requirements.

Clearly defined acceptance criteria help reviewers understand the expected behavior and determine whether the proposed solution meets the original requirements.

Step 2. Create a pull request or merge request

The developer submits the changes through the team's version control platform.

A useful pull request description includes the purpose of the change, important implementation details, associated work items, and testing information. Screenshots or reproduction steps can provide additional context when relevant.

This allows reviewers to understand the intended outcome before examining the modified files.

Step 3. Run automated quality checks

Automated checks evaluate the proposed changes for problems that can be detected through configured tools and tests.

Depending on the project, these may include unit tests, integration tests, linting, static analysis, security scanning, and build validation.

In a CI/CD workflow, these checks can run automatically when a pull request is created or updated, giving developers early feedback about potential issues.

Step 4. Assign reviewers and examine the changes

Reviewers are selected based on their knowledge of the affected codebase, technical domain, or application component.

They examine the modified files, understand relevant dependencies, and evaluate whether the implementation achieves its intended purpose.

Changes involving shared infrastructure, sensitive data, or critical application functionality may require additional specialized review.

Step 5. Provide feedback and request changes

Reviewers document concerns through inline comments, suggested modifications, or formal review decisions.

Useful feedback identifies the specific problem and explains why it matters. For example, a reviewer might flag a database query executed repeatedly inside a loop because its cost could increase as the dataset grows.

Comments requiring correction should be clearly distinguished from optional improvements.

Step 6. Address feedback and update the code

The author examines reviewer comments, makes necessary changes, and responds to questions.

Some feedback requires implementation changes, while other concerns may be resolved through clarification or technical discussion.

Revisions can trigger another round of automated validation and review.

Step 7. Approve and merge the changes

Once required issues are resolved and applicable quality checks pass, authorized reviewers approve the implementation.

The changes can then be merged according to the repository's configured rules, which may require specific approvals, successful checks, or resolved discussions.

After merging, the changes continue through the team's testing, release, and deployment procedures.

What should you look for during a code review?

A consistent code review checklist helps developers inspect changes systematically and focus on issues that could affect application behavior, security, or future maintenance.

The depth of review should reflect the risk and complexity of the change. A documentation correction requires different scrutiny from a modification to authentication logic.

Code review checklist for developers

Use the following checklist when examining a pull request or preparing changes for review.

• Code correctness: Does the implementation meet the requirements and produce the expected results?
• Edge cases: Does it handle missing inputs, invalid values, boundary conditions, and unexpected states?
• Readability: Are names, functions, and logical flows clear enough for another developer to understand?
• Security: Are authorization, input validation, sensitive data, and relevant trust boundaries handled appropriately?
• Performance: Could the change introduce excessive database queries, unnecessary operations, or resource consumption?
• Testing: Are relevant success, failure, and regression scenarios covered by appropriate tests?
• Maintainability: Does the implementation follow existing patterns and avoid unnecessary duplication or complexity?
• Documentation: Are affected APIs, configurations, technical decisions, or usage instructions updated where necessary?
• Change impact: Have interactions with existing components and potential backward-compatibility concerns been considered?

For example, when reviewing a change to an API response structure, developers should examine more than the modified endpoint. They may need to verify that existing consumers can continue processing the response or that any breaking change has been coordinated appropriately.

The checklist is a reusable starting point. Teams can extend it with domain-specific requirements, such as database migration safety, accessibility, privacy, or infrastructure security.

What does a code review look like? A practical example

Consider a hypothetical application that allows users to update the email address associated with their account.

A developer creates an API endpoint that receives an account ID and a new email address. The endpoint retrieves the account and saves the updated information.

The implementation appears to work during ordinary functional testing, but a reviewer identifies a missing authorization check.

Initial implementation: Missing authorization

The following simplified JavaScript example illustrates the relevant operations:

const accountId = req.params.accountId;

const account = await db.accounts.findById(accountId);

await db.accounts.updateEmail(
  account.id,
  req.body.email
);

The code retrieves an account using the identifier supplied in the request and updates its email address.

However, it does not verify whether the authenticated requester has permission to modify that account.

If the endpoint is reachable and no other authorization control prevents the operation, a user could potentially supply another account's identifier and modify its information.

Reviewer feedback: Verify account-level permissions

The reviewer identifies the missing authorization check as a blocking issue.

A useful review comment might read: The endpoint updates the account using the ID from the request, but I don't see an authorization check confirming that the current user can modify this account. Could we validate account-level permissions before the update and add a test covering an unauthorized request?

This feedback identifies the problem, explains the security concern, and specifies what needs to change.

Revised implementation: Validate permissions before updating

The developer revises the relevant logic:

const accountId = req.params.accountId;

const account = await db.accounts.findById(accountId);

if (!account) {
  return res.sendStatus(404);
}

if (!canEditAccount(req.user, account)) {
  return res.sendStatus(403);
}

await db.accounts.updateEmail(
  account.id,
  req.body.email
);

In this illustrative example, req.user represents the identity established by trusted server-side authentication, while canEditAccount() represents the application's authorization policy.

The updated logic checks whether the account exists and whether the authenticated requester is permitted to modify it before performing the update.

The example focuses on authorization. A production implementation would also need appropriate input validation, error handling, and any additional controls required by the account-update workflow.

Final review and approval

The developer adds tests confirming that:

• An authorized user can update the permitted account.
• An unauthorized user receives a forbidden response.
• A request for a nonexistent account follows the expected error-handling behavior.

The reviewer examines the updated implementation and verifies the relevant test results before approving the change.

The example demonstrates how code review can identify a security-sensitive problem even when the intended feature appears functional during ordinary testing.

What are the best practices for effective code reviews?

Effective code reviews depend on how developers prepare changes, communicate feedback, and manage approval responsibilities. Consistent working practices make it easier to maintain review quality without creating unnecessary delays.

The following eight code review best practices provide a practical starting point.

1. Keep code changes small and focused

Large pull requests require reviewers to understand more context and evaluate more interactions between changes.

Where practical, separate unrelated modifications and organize complex features into smaller, logically connected changes. This makes individual reviews easier to understand and allows developers to identify the source of problems more precisely.

2. Provide clear context and descriptions

A reviewer should understand the purpose of the change before examining the implementation.

Include the relevant requirement, the reasoning behind important technical decisions, testing instructions, and any known limitations. Context becomes especially important when the change affects several services or introduces unfamiliar behavior.

3. Review your own code before submitting

Examine the complete diff before requesting another developer's time.

Look for accidental file changes, temporary debugging statements, unused code, unclear naming, and missing tests. A self-review can resolve straightforward problems and make the subsequent peer review more productive.

4. Establish clear review guidelines

Document expectations for reviewer responsibilities, approval requirements, and the conditions that determine whether a change is ready to merge.

Guidelines should also explain how teams handle urgent fixes, high-risk changes, and disagreements over implementation choices.

These shared expectations help developers apply consistent standards across repositories and projects.

5. Distinguish blocking issues from optional suggestions

Review comments should communicate the importance of the requested change.

A useful classification is:

For instance, a missing permission check may create a security risk that prevents approval, while a minor naming preference may be left to the author's judgment.

Teams should document these categories so reviewers apply them consistently. Actual merge restrictions still depend on the repository's configured approval and protection rules.

6. Prioritize timely and focused reviews

Review requests can accumulate when developers have competing responsibilities or when ownership of a component is unclear.

Establish reasonable response expectations and identify reviewers early, particularly for changes affecting critical components.

Engineering managers should also consider review responsibilities when planning development capacity so that reviewing colleagues' work receives appropriate attention.

7. Automate repetitive quality checks

Automated formatting, linting, testing, and static analysis can handle many routine checks before a human reviewer examines the implementation.

This gives developers more time to consider behavior, architecture, security-sensitive decisions, and interactions between components.

The goal is to create a dependable review workflow where automation and human judgment address different categories of concerns.

8. Encourage constructive technical discussions

Code reviews frequently involve alternative approaches that are technically valid.

Reviewers should explain the reasoning behind their concerns, while authors should be able to discuss trade-offs and provide additional context.

When a discussion reveals an important architectural decision, recording the agreed approach can help future contributors understand why the implementation was chosen.

What are the common challenges in code reviews?

Code review practices can become inefficient when approval responsibilities are unclear, changes are difficult to evaluate, or technical feedback requires repeated clarification.

These problems can affect both development timelines and the consistency of review decisions.

Approval bottlenecks deserve particular attention.

Suppose one senior engineer must review every change to a shared service. As the number of contributors grows, that engineer may become a recurring constraint on development progress.

The team could address the problem by improving documentation, distributing knowledge of the component, and developing additional qualified reviewers. Changes involving sensitive functionality can still retain the specialized approval requirements they need.

For engineering managers, recurring review delays may reveal broader issues with team ownership, knowledge distribution, or development capacity.

What tools are used for code reviews?

Code review tools support different activities, including examining proposed changes, recording feedback, identifying potential defects, and coordinating engineering work.

Teams often combine several tools because repository reviews, automated quality checks, and delivery tracking serve different purposes.

1. Version control and pull request review tools

Platforms such as GitHub, GitLab, Bitbucket, and Gerrit provide mechanisms for reviewing proposed code changes.

Depending on the platform and configuration, teams can inspect code differences, leave inline comments, request changes, manage reviewer assignments, and enforce approval requirements.

These platforms commonly serve as the primary environment for managing code-level review discussions and decisions.

2. Static analysis and automated quality tools

Static analysis tools examine source code without running the application.

Tools such as SonarQube and language-specific linters can identify certain coding errors, security concerns, and maintainability issues.

They complement automated tests, which execute code to evaluate specific behavior under defined conditions.

3. AI-assisted code review tools

AI-assisted review tools can analyze proposed changes, summarize code modifications, suggest improvements, and highlight potential problems.

These capabilities can be particularly useful when developers need help examining unfamiliar code or understanding a large set of changes.

However, generated findings may be incomplete or incorrect, particularly when they depend on business requirements, system architecture, or context outside the available code.

Human reviewers should verify AI-generated suggestions and evaluate their relevance before making approval decisions.

4. Project and engineering workflow management tools

Code review also affects how development work progresses through a project.

A feature may be implemented but still waiting for approval, a security fix may require additional verification, or a review may reveal work that needs to be addressed in a separate task.

Project management platforms such as Plane help engineering teams organize work items, assign ownership, manage dependencies, and track progress across development workflows.

For example, teams can use bug tracking workflows in Plane to coordinate reported defects, resolution work, and verification activities while conducting code-level reviews in their repository tools.

This provides a clearer view of how review-related work contributes to overall project delivery.

How do you measure the effectiveness of code reviews?

Code review metrics help engineering teams understand how changes progress through review, where delays occur, and whether recurring quality issues need attention.

Teams should define measurement boundaries consistently because review practices and repository workflows vary.

Five useful metrics are outlined below.

1. Time to first review

Time to first review measures how long a change waits between requesting review and receiving the first meaningful reviewer response.

A consistently long waiting period may indicate limited reviewer availability, unclear ownership, or competing development priorities.

2. Code review cycle time

Code review cycle time measures the time spent completing the review process.

For example, a team might measure the period from the first substantive review to final approval, including the time required for revisions and subsequent feedback.

The team should document its chosen start and end points to keep comparisons meaningful.

3. Pull request turnaround time

Pull request turnaround time measures the elapsed time from opening a pull request to merging it.

This broader measurement can include waiting for initial review, revision cycles, automated checks, dependencies, and merge scheduling.

Comparing turnaround time with review-specific measures helps teams determine where delays occur.

4. Review rework and iteration rate

Review iterations indicate how many rounds of feedback and revision a change requires before approval.

Repeated cycles can reveal unclear requirements, disagreements about implementation, or insufficient preparation.

High iteration counts warrant investigation, although complex changes may reasonably require several rounds of technical discussion.

5. Post-merge defects

Post-merge defects are problems discovered after code has entered the shared codebase.

Examining these issues helps teams understand which problems escaped their existing review and testing practices.

When defects recur, teams can use root cause analysis to identify contributing factors and determine whether review procedures need improvement.

Code review metrics comparison

The value of these metrics comes from examining them together.

For example, a team might reduce review turnaround time by approving changes faster, while post-merge defects increase. That pattern would warrant investigating whether review quality, test coverage, or change complexity has shifted.

Review measurements can also contribute to a broader understanding of engineering velocity, particularly when approval delays affect the movement of work from implementation to release.

Teams should use these metrics to improve workflows and identify systemic constraints rather than evaluating individual developers solely on review speed or comment volume.

Closing thoughts

Code reviews help engineering teams examine implementation decisions, identify potential defects, and maintain consistent quality standards before changes enter the shared codebase.

A reliable review process starts with clear requirements, manageable changes, knowledgeable reviewers, and actionable feedback. Automated checks can handle routine validation, while human reviewers examine application behavior, technical decisions, and risks that require additional context.

As development teams grow, maintaining clear ownership and monitoring review bottlenecks becomes increasingly important. Together, these practices help teams make code reviews a consistent part of delivering maintainable and reliable software.

Frequently asked questions

Q1. What is the main purpose of a code review?

The main purpose of a code review is to examine proposed source code for correctness, security, quality, and maintainability before it is merged into a shared codebase. Reviews also help developers share technical knowledge and apply consistent engineering standards.

Q2. Who is responsible for conducting code reviews?

Code reviews are usually conducted by developers familiar with the relevant programming language, component, or codebase. Engineering leads may assign reviewers, while repository rules can require approval from designated code owners or specialists for particular changes.

Q3. What is the difference between code review and code testing?

Code review involves examining source code and implementation decisions to identify potential issues. Code testing involves executing software to verify its behavior against expected outcomes. Both practices support quality assurance and can identify different kinds of problems.

Q4. When should code reviews be performed?

Code reviews commonly take place after a developer prepares a change and before it is merged into a shared branch. Teams may also review code during development through pair programming or synchronous walkthroughs. The timing depends on the development workflow and approval requirements.

Q5. Can AI replace manual code reviews?

AI-assisted tools can identify potential issues, summarize changes, and recommend improvements. However, their findings require verification, especially when reviewing business logic, security requirements, and architectural decisions. Human oversight remains important for understanding context and approving changes.