This handbook supports the quality auditor Body of Knowledge (BoK), developed for the ASQ Certified Quality Auditor (CQA) program. This fourth edition addresses new and expanded BoK topics, common auditing (quality, environmental, safety, and so on) methods, and process auditing. It is designed to provide practical guidance for system and process auditors. Practitioners in the field provided content, example audit situations, stories, and review comments as the handbook evolved. New to the fourth edition are the topics of common and special causes, outliers, and risk management tools. Besides the new topics, many current topics have been expanded to reflect changes in auditing practices since 2004 and ISO 19011 guidance, and they have been rewritten to promote the common elements of all types of system and process audits. The handbook can be used by new auditors to gain an understanding of auditing. Experienced auditors will find it to be a useful reference. Audit managers and quality managers can use the handbook as a guide for leading their auditing programs. The handbook may also be used by trainers and educators as source material for teaching the fundamentals of auditing