The Transparent Prolog Machine (TPM) was one of the most successful products of the UK's Alvey programme. Developed at the Open University, UK, in collaboration with Expert Systems International, TPM is a new approach to debugging Prolog programs and to teaching the Prolog language. This book will be of particular value to Prolog programmers in Artificial Intelligence, Computer Scientists in AI, human-computer interaction and graphics researchers and practitioners. The style of graphical presentation developed within TPM (AORTA diagrams) is now used not only in TPM software but also in Open University teaching materials for Prolog; it is widely accepted as the best graphical visualization of Prolog execution. The book presents both the theory behind the TPM graphical debugger and a detailed account of its operation, including worked examples, code abstractions, and a tutorial.