The Dunedin Longitudinal Study is the longest running study of mental health in the world; Study members have participated in psychiatric interviews since they were 11 years old, and their lives span all editions of the Diagnostic and Statistical Manual of Mental Disorders, since the advent of DSM-III. We pair the Dunedin Study with the E-risk Study to gain insights about the etiology of mental health problems, to provide practical information about prevention and prognosis, and to generate new knowledge about the measurement and classification of psychiatric disorders.