Code review is a systematic software quality assurance technique where developers examine each other's code changes before merging them into the main codebase. Originating from formal inspection processes in the 1970s and evolving through pair programming and modern pull request workflows, code reviews now serve as the linchpin of collaborative software development—catching bugs early, spreading knowledge across teams, and maintaining code health over time. The practice has become so fundamental that research shows teams practicing thorough code review ship 15% fewer bugs to production, yet the difference between effective and performative review often comes down to a handful of non-obvious principles: understanding when to block versus when to nitpick, how cognitive load affects defect detection rates, and why smaller PRs paradoxically receive more substantive feedback than large ones.