In this article it is proposed that the principles of motivational intensity theory (Brehm & Self, 1989) apply to effort mobilization for challenges with consequences for performers' self-esteem and selfdefinition (i.e., self-involvement). Accordingly, involvement of the self makes success important and thus justifies the mobilization of high resources. However, up to this level of maximally justified resources, actual effort is mobilized in correspondence to subjective task difficulty as long as success is possible. We report a series of experimental studies that have operationalized effort intensity as cardiovascular reactivity during task performance and used multiple manipulations of self-involvement (social evaluation, self-awareness, ego involvement, personal goals) and task difficulty. The empirical evidence clearly supports the idea that the principles of motivational intensity theory apply to performance conditions that have direct consequences for self-definition and self-esteem and challenges a number of other theoretical accounts.