This book focuses on the essential concepts of programming languages. The text's analytic and hands-on approach brings the reader face-to-face with the most fundamental idea in computer programming-The interpreter for a computer language is just another program. As a programmer, mastering the idea of an interpreter for a computer language is a source of great power. It provokes a real shift in mindset, a basic change in the way one thinks about programming. Essentials of Programming Languages uses interpreters, written in Scheme, to express the semantics of many essential language elements in a way that is both clear and executable. It also examines some important program analyses. Extensive exercises explore many design and implementation alternatives. This book is a mandatory tool for anyone aspiring to become a designer of languages, rather than being a mere user. It enables the reader to choose the rules by which languages are put together, rather than follow the rules chosen by others.