Most of the critical issues related to semantics, or meaning, of program elements, and the text uses interpreters (short programs that directly analyze an abstract representation of program text) to express the semantics of many essential language elements in a way that is both clear and enforceable. This book provides a view of programming languages using widely varying levels of abstraction, maintaining a clear link between high-level and low level views. Complete Scheme code for all interpreters and analysis in this book can be found online through the MIT Press website. For this new edition, each chapter has been revised and new exercises that have been added.