API authentication is a crucial mechanism for verifying user credentials against predetermined rules to ensure access to protected resources. There are various approaches, each with its own security, complexity, and maintainability trade-offs. Basic authentication involves sending a username and password in the authorization header as a Base64-encoded string, but it lacks built-in expiration or rotation features and depends on transport-layer encryption (HTTPS). API keys act as simple shared secrets transmitted with each request, commonly sent via a custom header, offering flexibility but lacking fine-grained scopes or expiration. Bearer tokens are short-lived credentials distributed by an identity provider or authentication service, restricting token lifetimes to restrict token leaks. OAuth 2.0 is a framework offering a structured approach to token-based authorization, commonly used for delegated authorization and enabling applications to access user data without storing passwords. JWTs (JSON Web Tokens) are self-contained tokens that incorporate authentication and authorization claims within an encoded structure, removing the need for server-side sessions. HMAC signatures calculate a cryptographic hash by combining request data with a shared secret, offering tamper-proofing data in high-integrity environments. Session cookies maintain session data keyed by an identifier, sent to the client as a cookie, but can violate stateless REST principles and add server-side session management overhead. Each approach has unique implications for security, scalability, and user experience, requiring best practices such as using encrypted connections, frequent credential rotation, principle of least privilege, secure secret storage, auditing authentication logs, and careful key management to ensure effective API authentication.