According to the Model of Human Occupation (MOHO), beliefs regarding competency can influence whether a person’s occupational role behavior is adaptive or maladaptive. Such beliefs are considered to be part of a person’s sense of “personal causation.” This article reviews some of the theoretical underpinnings of the personal causation construct. Issues addressed are the distinction between competency beliefs and locus of control (another aspect of personal causation according to the MOHO); the domain-specific nature of competency beliefs; and, in particular, the evidence for a relationship between competency beliefs and actual behavior. The article focuses on competency beliefs and their relationship to three domains of occupational behavior that have relevance for adolescents: academic ability, social competence, and physical competence. Implications for clinical practice with adolescents with psychiatric disorders are addressed.