What accounts for the formation and disintegration of social movement alliances? The dominant approach in social movement studies stresses the role of political opportunities and threats in facilitating or undermining alliances between oppositional groups. This article argues, by contrast, that the convergence and divergence of contenders’ perceptions mediate between political opportunities and shifting alliances. Whereas previous studies conceptualize perceptions as global assessments of actors’ environments, I disaggregate three dimensions of the concept: optimism about state elites, optimism about state institutions, and optimism about contentious collective action. The Iranian Reform Movement of 1997 to 2005 offers a nearly ideal case for the study of perceptions and alliances, because it encompasses a variety of opposition groups whose alliances formed and disintegrated over the course of the movement’s rise and decline. This article examines shifting perceptions of opportunity among these groups and documents how these perceptions affected alliances, independent of state repression and groups’ ultimate goals.