The theory of the firm contained in most textbooks is inadequate for understanding the economic behavior of cooperatives because some assertions about firm behavior, such as profit maximization, may be inappropriate for cooperatives. This article provides an introduction to the neoclassical theory of cooperatives, which has been useful for generating insights into the behavior of cooperatives in various market structures, helping cooperatives develop business strategies consistent with their objectives, and informing public policy decisions concerning cooperatives. Part I of this article presents the basic elements of the neoclassical theory as it pertains to farm supply cooperatives.