API key authentication is a straightforward method for applications and services to authenticate requests to an API. A client receives a unique key—typically a long, randomly generated string—which is included in API requests through headers, query parameters, or request bodies. The server validates this key against its database to confirm the requester's identity and permissions before processing the request.

This authentication approach is widely used in REST APIs, webhooks, and MCP (Model Context Protocol) integrations because it's simple to implement and manage. Unlike OAuth, API keys don't require complex token exchanges; however, they're generally less secure for user-facing applications since the key must be stored on the client side. API keys are ideal for service-to-service communication, development environments, and scenarios where a fixed identity needs persistent access.

Best practices include rotating keys regularly, restricting key scope to necessary permissions, storing keys securely in environment variables or secrets management systems, and revoking compromised keys immediately. Many platforms allow multiple keys per user or service, enabling granular access control and easier key rotation without service interruption.