Despite the lack of scientific consensus about the etiologies of mental disorders, practicing clinicians and laypeople alike hold beliefs about the causes of mental disorders, and about the causal relations among symptoms and associated characteristics of mental disorders. This chapter summarizes research on how such causal knowledge systematically affects judgments about the diagnosis, prognosis, and treatment of mental disorders. During diagnosis, causal knowledge affects weighting of symptoms, perception of normality of behaviors, ascriptions of blame, and adherence to the DSM-based diagnostic categories. Regarding prognosis, attributing mental disorders to genetic or neurobiological abnormalities in particular engenders prognostic pessimism. Finally, both clinicians and laypeople endorse medication more strongly as an effective treatment if they believe mental disorders are biologically caused rather than psychologically caused. They also do so when considering disorders in the abstract versus equivalent concrete cases. The chapter discusses the rationality, potential mechanisms, and universality of these phenomena.