What is Code Review?
Code review is the systematic examination of source code by peers before it is merged into the main codebase.
Code review is the systematic examination of source code by peers before it is merged into the main codebase. It is one of the most effective quality assurance practices in software engineering, catching bugs, enforcing standards, and spreading knowledge across the team.
Modern code review happens through pull requests (PRs) or merge requests (MRs) on platforms like GitHub, GitLab, or Bitbucket. A developer submits their changes, one or more reviewers examine the diff, leave comments, request changes, and eventually approve the merge.
Effective code reviews catch 60-90% of defects that automated testing misses. They also serve as knowledge transfer — junior developers learn patterns from senior reviewers, and senior developers stay aware of codebase changes they didn't write.
Google's research shows that code review effectiveness drops sharply after 200 lines of code. Smaller, more frequent reviews are significantly more effective than large batch reviews.
Why It Matters
Code review is the frontline defense against technical debt. Every code change that introduces a shortcut, violates a pattern, or lacks tests is an opportunity for a reviewer to catch it before it compounds. Teams without code review accumulate debt 2-3x faster.
How to Measure
1. **Review Turnaround Time**: Time from PR submission to first review. Target: <4 hours.
2. **Review Coverage**: % of code changes that receive review. Target: 100%.
3. **Comments Per Review**: Average feedback density. Too low (<1) suggests rubber-stamping.
4. **Rejection Rate**: % of PRs that require changes. 20-40% is healthy.
Frequently Asked Questions
How long should a code review take?
Reviewing 200 lines should take 30-60 minutes. Larger reviews should be broken into smaller PRs. Google research shows effectiveness drops sharply after 200 lines.
What should code reviewers look for?
Logic errors, security vulnerabilities, test coverage, code style consistency, performance issues, documentation, and architectural alignment.
Related Terms
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