API authentication is the process of verifying the identity of clients making requests to an API, forming the foundation of API security in modern distributed systems. Unlike traditional web authentication that relies on session cookies, API authentication must work across stateless HTTP requests, supporting diverse clients from web browsers to mobile apps and server-to-server integrations. The challenge lies in balancing security with developer experience—while robust authentication prevents unauthorized access and data breaches, overly complex implementations create friction that slows development and adoption. Understanding when to use API keys for server identification, OAuth for user delegation, JWTs for stateless verification, or modern passkeys for passwordless authentication is critical for building secure, scalable APIs.