Authentication and authorization are critical for securing access to your Web API's resources. In ASP.NET Core Web API, you can implement authentication and authorization by using authentication middleware and authorization policies. Authentication middleware, such as JWT bearer authentication or OAuth 2.0, validates the identity of the client. Authorization policies, defined using attributes or policy-based approaches, control access to specific resources or actions based on roles, claims, or other criteria. You can integrate with external authentication providers like Azure AD, Google, or Facebook using authentication middleware. Implementing authentication and authorization ensures that only authenticated and authorized users can access your Web API.