In this paper, I examine the role that irrigation played in the formation of the Southern Moche state in the Moche Valley, Peru. Specifically, I attempt to test Wittfogel and Steward's hydraulic model, which postulates that in certain arid environments, the managerial requirements of construction and maintenance of irrigation systems played a crucial role in the formation of centralized polities. I formulate and evaluate four hypotheses concerning the role of irrigation systems in the Moche Valley. Those hypotheses are then evaluated using settlement pattern data drawn from two surveys that cover the entire coastal section of the valley and provide information on 910 archaeological sites. Based on those data, I present a sequence of political development for the valley from the formation of the first autonomous village in the Late Preceramic period (2500–1800 B. C.) to the zenith of the Southern Moche state. Evaluation of the four hypotheses indicates that the managerial requirements of irrigation were relatively unimportant; rather, warfare, highland-coastal interaction, and political control of irrigation systems created opportunities for leaders to form a highly centralized, territorially expansive state sometime between A. D. 200 and 700.