This book describes the implementation of Icon in detail. It received uniformly good reviews, with comments such as "a classic" and "a fascinating read". Highlights include: * Icon's virtual machine * the interpreter for the virtual machine * generators and goal-directed evaluation * data representation * string manipulation * structures * memory management The book describes the implementation of Version 6 of Icon. Most of the material in the book still is relevant. Two technical reports supplement the book to describe subsequent changes. The implementation of Icon's graphics facilities is not described.