The book is written with the objective of automating the audit decision in detecting variations or exceptional data between the current and preceding month in payroll processing, using an Expert System. Expert systems are in the category of Artificial Intelligence (AI) application. They are now in the class of electronic spreadsheets and are commercially available and affordable having been in existence since the 1970s. The necessary steps to the development of an expert system have been covered. The payroll audit decision expert system is therefore highly commended to Auditors or Accountants seeking to detect and prevent Payroll fraud through an Expert System.