Economists and historians have struggled for almost 80 years to account for the American Great Depression, which began in 1929 and lasted until the early years of World War II. In this article, the authors discuss three major schools of thought on the causes of the Great Depression and the long failure of the American economy to return to full employment: (1) "The Keynesian Explanation"; (2) "The Monetarist Explanation"; and (3) "The International Explanation". The authors believe that teachers will find that presenting the approaches described in this article is a useful way of encouraging students to review and analyze the causes and progression of the Great Depression. By examining the known historical facts and developing their own interpretations, students can further their understanding both of the history of the Great Depression and of some basic principles of economics.