Backend API testing validates the functionality, reliability, security, and performance of application programming interfaces that power modern distributed systems. Unlike UI testing, API testing operates at the business logic layer, validating data contracts, response structures, and integration points before frontend code ever touches them. A robust API testing strategy combines unit tests for isolated business logic (70% of your suite), integration tests for endpoint behavior (20%), and end-to-end tests for critical workflows (10%), following the widely accepted testing pyramid model. The key mental model: test APIs as contracts between services—any breaking change to request/response structure, authentication flows, or error semantics can cascade failures across dependent systems, making comprehensive testing essential for maintaining system reliability and developer velocity.