A number of comparative judicial scholars have argued that assertions of judicial authority are a function of the level of fragmentation/competition in the formal political sphere. Accordingly, in authoritarian or one-party settings, judges should be deferential to power holders, and in places where political power is divided between branches and/or parties, one would expect to see greater levels of judicial assertiveness. Through a longitudinal, qualitative analysis of one most-likely case (Chile) and one least-likely case (Franco-era Spain) and drawing on a half-dozen other cases from the comparative judicial literature, this article argues that political fragmentation is neither sufficient nor necessary for judges to challenge powerful actors. Instead, it argues that assertive or “positively independent” judicial behavior requires ideational support, in the form of a role conception/professional ideology that gives judges motivation for such behavior. Such professional attitudes are socially and institutionally constituted in a dynamic process that itself shapes judges' perceptions of the opportunities for and obstacles to judicial assertiveness, both within and outside the judiciary.