In this paper, we examine how the past alliance behavior of nations affects the likelihood that these states will be involved in alliance formation. We contend that nations evaluate the reputations of potential allies when searching for alliance partners. Reputation information is processed by governments along with other immediate concerns. By introducing a model and developing subsequent measures of reputational alliance histories, we improve upon our current understanding of the factors that drive alliance formation. Using alliance reputation data derived from the ATOP project (1816β2000), we find support for the hypothesis that a reputation for upholding one's agreements significantly improves the likelihood of membership in future alliances