This book explains a new approach to debugging Prolog programs and to teaching the Prolog language. TPM's style of graphical representation (AORTA diagrams) is widely accepted as the best graphical visualization of Prolog execution. TPM was developed at the Open University in collaboration with Expert Systems International. The widely accepted presentation is used in the TPM software and also in the Open University teaching materials for Prolog. This book gives both the theory behind the TPM graphical debugger and a detailed account of its operation, including worked examples, code abstractions and a tutorial.