The ASP.NET Web API is a new framework for creating HTTP based "web apis".While using some traditional ASP.NET MVC concepts, such as controllers and routing, it also introduces new features such as support for multiple media types, content negotiation, TAP (Task-based Asynchronous Pattern) support and multiple hosting options (e.g. ASP.NET pipeline hosting and self-hosting).This session aims to provide a clear understanding of the ASP.NET Web API internal runtime architecture, showing the message path since requests are received to when responses are sent.It addresses the following subjects: hosting; HTTP request and response representation; extending the message path via message handlers and action filters; routing; controller and action selection and invocation; model binding; formatting and content negotiation; dependency injection and service location.This theoretical framework is complemented by a set of concrete examples, showing how to take advantage of this knowledge.