What is IdentityServer and When Do You Need it?

This article clarifies IdentityServer, a .NET SDK for building custom OAuth 2.x/OpenID Connect/SAML identity providers. It contrasts IdentityServer with simpler solutions like ASP.NET Identity, explaining that IdentityServer is middleware requiring custom C# code for startup, client registrations, user stores, and login UI. The article details supported protocols (OAuth 2.0/2.1, OIDC, FAPI 2.0, DPoP, etc.), licensing (paid for production), and extensibility. It emphasizes that IdentityServer is unnecessary for apps with only local users or external login via Google, recommending ASP.NET Core's AddOpenIdConnect() instead. The piece is a technical guide for developers evaluating authentication architectures in .NET.