APIs evolve. You need a versioning strategy:
URL versioning: /api/v1/users. Clear and explicit. Changing versions requires URL changes.
Header versioning: Accept: application/vnd.api+json;version=1. Cleaner URLs but less visible.
Query parameter: /api/users?version=1. Simple but pollutes the URL.
Best practices: Support - versions simultaneously. Deprecate with warnings. Give clients - months to migrate. Document breaking changes clearly.