The present paper focuses on the trait domain of conscientiousness, its development, and its relation to psychopathology. The definition of conscientiousness and its interface with specific forms of psychopathology, such as externalizing psychopathology, are discussed first. Then an overview of the known developmental changes in conscientiousness across the life course is provided, as well as the potential shared developmental etiology of conscientiousness and externalizing psychopathology. Given the lack of data, testing etiological factors that may contribute to the development of both conscientiousness and externalizing disorders, we report on three studies examining the childhood experiences associated with both conscientiousness and externalizing behaviors. Last, future directions are discussed.