Choice Outstanding Academic Title, 2009. Hundreds of programming languages used today? Scripting languages for Internet commerce, user interface programming tools, spreadsheet macros, page format specification language, and many others. Designing a programming language is a metaprogramming activity that bears certain similarities to programming in a regular language, with clarity and simplicity even more important than regular programming. It takes as premise and starting point the idea that when language behaviors become very complex, the description of behavior should be very simple. This book presents a set of tools (a mathematical metalanguage, abstract syntax, operational and denotational semantics) and use it to explore a set of programming language design dimensions, including dynamic semantics (naming, state, control, data), static semantics (types, type reconstruction, polymporphism, effects), and pragmatics (compilation, garbage collection).