Authentication and authorization are the backbone of secure APIs. Understanding their differences is crucial for implementing effective security strategies. Authentication verifies a user's identity, typically through credentials or biometric data, while authorization assigns permissions and access rights based on user roles and attributes. To build robust API authentication systems, developers should implement multi-factor authentication (MFA), use OAuth 2.0 and OpenID Connect protocols, enforce strong password policies, utilize token-based authentication mechanisms, secure communication channels with HTTPS, and regularly review authorization rules. On the other hand, to strengthen API authorization for maximum security, organizations should adopt role-based access control (RBAC) and attribute-based access control (ABAC), implement the principle of least privilege, use API gateway policies, and avoid common mistakes such as weak authentication mechanisms, over-permissive authorization, poor token management, ignoring regular security audits, and lack of granular access control. By following these best practices, developers can ensure their APIs are secure and protected from malicious attacks.