ASP.NET Core is a modular framework. This is made available by the ASP.NET Core middleware components. ASP.NET Core handles requests and responses via a set of operations that are completed by something called the request pipeline. This pipeline comes predefined with specific components, but, when needed, developers can remove components, add different built-in ones, or write their own.

Other components of the ASP.NET Core Framework that are not a part of the request pipeline, such as the dependency injection service, can also be replaced within the Startup class.