This chapter provides an overview of research examining the effects of people’s theories about willpower on health and subjective well-being. It describes that implicit theories about willpower are an important moderator of the effect of self-regulatory demands on self-regulatory behavior in everyday life and therefore predict important outcomes such as physical health and subjective well-being. Although people in their everyday life are not aware of their beliefs about willpower, they are able to report their theories when explicitly asked with targeted questions. Job and colleagues developed a questionnaire to assess willpower theories with respect to two commonly studied self-control domains: strenuous mental activities and resisting temptations. One may think that what people think about willpower simply reflects how they perceive their own self-control capacity. People low in trait self-control might think of willpower as limited because they frequently fail in self-control attempts.