What really happens when a program runs? Essentials of Programming Languages teaches the fundamental concepts of programming languages through numerous short programs, or interpreters that actually implement the features of a language. Nearly 300 exercises using these programs provide a hands-on undertanding of programming principles that is hard, if not impossible, to achieve by formal study alone. In an approach that is uniquely suited to mastering a new level of programming structure, the authors derive a sequence of interpreters that begins with a high-level operational specification (close to formal semantics) and ends with what is effectively assembly language - a process involving programming transformation techniques that should be in the toolbox of every programmer.