The central purpose of this article is to produce a model that will explain both fluctuations and growth through the operation of endogenous forces. It thus attempts a synthesis of business cycle theories of the Tinbergen type and growth theories of the Harrod type. The model stresses those factors that, in the opinion of the author, explain the great depression of the 1930's and the continued growth (subject to minor fluctuations) of the economy since the war. It does not seek to explain the minor fluctuations that have occurred in the last decade. This article has been arranged, at the cost of some repetition, to meet the requirements of non-mathematical and mathematical readers.