This article explores the place of game theory in Elinor Ostrom's groundbreaking studies of nonmarket institutions for managing common-pool resources (CPRs) from the 1980s onward. While game theory had been associated with the study of CPRs since at least the later 1960s, the precise way scholars used the theory varied, depending not only on their home discipline but on the locally specific research program they hoped to develop. Ostrom's use of game theory ultimately looked quite different from that of laboratory psychologists studying “commons dilemmas,” political scientists theorizing about constitutional choice, or economists building formal models of sparse markets, bearing perhaps the closest resemblance to the economist Reinhard Selten's skeptical and heterodox applications of game theory to the study of industrial organization. This account of Ostrom's local adaptation of game theory potentially helps us evaluate critiques of her work that focus on her use of rational choice models.