How do you implement semantic versioning for libraries and APIs?

Semantic versioning (SemVer): MAJOR.MINOR.PATCH. MAJOR: breaking changes. MINOR: backward-compatible features. PATCH: backward-compatible fixes. Breaking changes: removed APIs, changed signatures, changed behavior. Process: document changes in CHANGELOG, tag releases, use pre-release versions (1.0.0-alpha), protect main branch. API evolution: deprecate before removal, version APIs, provide migration guides. Tools: semantic-release (auto-versioning from commits), conventional commits. Consider: API stability promise, dependency management implications. Clear versioning builds user trust.