The book is written with the objective of automating the audit decision in detecting variations or exceptional data between the current and preceding or penultimate month in payroll processing, using an Expert System. The book begins in Chapter one with an introduction. Chapter two is a review of the expert system theory and empirical literature on its use in business applications while Chapter three presents the methodology of research. Chapter Four dwells on the design and development of the expert system software for payroll audit. Finally, Chapter Five concludes with a summary and recommendations. The payroll audit decision expert system is therefore highly commended to end-users such as internal or external auditors, accountants, fraud examiners, risk consultants and enthusiastic readers seeking to detect and prevent Payroll fraud through an Expert System. The book is also written for the consumption of interested Expert System researchers.