C++20 is the next big C++ standard after C++11. As C++11 did it, C++20 changes the way we program modern C++. This change is, in particular, due to the big four of C++20: ranges, coroutines, concepts, and modules. My book C++20 is a tutorial and a reference for the C++20 standard. It teaches you C++20 and provides details of this new, thrilling C++ standard. The exciting factor is mainly due to the big four of C++20. As the C++20 standard is brand-new, many examples can only be compiled and executed with a specific compiler. I use the newest GCC, Clang, and MSVC compilers. Coroutines are generalized functions that can be suspended and resumed later while maintaining their state. Coroutines are a convenient way to write event-driven applications. Event-driven applications can be simulations, games, servers, user interfaces, or even algorithms. Coroutines are also typically used for cooperative multitasking. C++20 does not provide concrete coroutines, but C++20 provides a framework for implementing coroutines. This framework consists of more than 20 functions, and some of which you must implement, some of which you can override. Therefore, you can tailor coroutines to your needs.