Mental disorders are one main focus of research interest in the 31 year study of the Northern Finland 1966 Birth Cohort Study. Mental disorders are quite common in young adulthood and they have a great impact on quality of life and working ability. Good national registers in Finland ensure the possibility to follow up treated incidence of severe mental disorders. On the other hand, a notable part of those who suffer from non-psychotic mental disorders do not receive any psychiatric treatment. That is why it is not possible to follow up psychiatric morbidity of the non-psychotic disorders from register data. In this review, principles of psychiatric diagnostics, known prevalences of psychiatric disorders in population and factors connected with mental disorder are briefly presented. Especially childhood predictors of mental disorders are reviewed.