This paper reports two studies that investigated children's conceptions of mental illness using a naive theory approach, drawing upon a conceptual framework for analysing illness representations which distinguishes between the identity, causes, consequences, curability, and timeline of an illness. The studies utilized semi-structured interviewing and card selection tasks to assess 6- to 11-year-old children's conceptions of the causes and consequences (Study 1) and the curability and timeline (Study 2) of different mental and physical illnesses/ailments. The studies revealed that, at all ages, the children held coherent causal-explanatory ideas about the causes, consequences, curability, and timeline of both mental and physical illnesses/ailments. However, while younger children tended to rely on their knowledge of common physical illnesses when thinking about mental illnesses, providing contagion and contamination explanations of cause, older children demonstrated differences in their thinking about mental and physical illnesses. No substantial gender differences were found in the children's thinking. It is argued that children hold coherent conceptions of mental illness at all ages, but that mental illness only emerges as an ontologically distinct conceptual domain by the end of middle childhood.