This section presents a high-level overview of the asynchronous model at the core of the Boost.Asio library. This model enshrines asynchronous operations as the fundamental building block of asynchronous composition, but in a way that decouples them from the composition mechanism. The asynchronous operations in Boost.Asio support callbacks, futures (both eager and lazy), fibers, coroutines, and approaches yet to be imagined. This allows the application programmer to select an approach based on appropriate trade-offs.