Over the past 50 years, a number of different approaches or models have represented AIS. Each new model evolved because of the shortcomings and limitations of its predecessor. An interesting feature in this evolution is that the newest technique does not immediately replace older models. Thus, at any point in time, various generations of systems exist across different organizations and may even coexist within a single enterprise. The modern auditor needs to be familiar with the operational features of all AIS approaches that he or she is likely to encounter. This book deals extensively with five such models: manual processes, flat-file systems, the database approach, the REA (resources, events, and agents) model, and ERP (enterprise resource planning) systems. Each of these is briefly outlined in the following section.