The Resource, Events, and Agents (REA) model has been the subject of lively research for over 40 years since it was first developed by McCarthy (1982). That seminal paper has been cited over a thousand times, and the REA model is now the basis for many operational data bases. Our objective in this educational paper is to explain in simple terms what REA is, why it was created, and how it works. We return to the genesis of REA in McCarthy (1979, 1982) and place those papers in the context of accounting and computer technology developments at the time. We then define REA rather than relying on examples, and we explain concepts like duality and ontology that many readers find especially difficult. The audience for this paper is those who recognize the importance of REA but lack access to expert teachers who can help them learn it. JEL Classifications: M2; M4; M41.