FastAPI became one of the most web frameworks in Python. It has an amazing documentation, and easy to use API, which made it very popular. It's easy to start, and as a developer you have a lot of power on what you can do. But... How does it work internally? In this talk, we will explore the internals of FastAPI. We'll explore the dependency injection system, what are the benefits, and limitations. We'll also see how the routing system works, when the middleware stack runs, how the request and response are handled in detail, how the OpenAPI schema is generated, and the differences between async and non-async endpoints, and how WebSockets fit in the whole picture. Furthermore, we'll also see how the dependencies Pydantic and Starlette help FastAPI on its job. At the end of this talk, the attendee will understand what's underneath of this very popular package.