This chapter reviews sociological theories and measurements of stratification and social class in the process of relating inequality to mental disorder. It reviews the major research studies, noting differences in indicators of social class and methodological variations. Mental disorders can have infectious, toxicologic, genetic, neurodevelopmental, psychological, and social causes. Adolescence and young adulthood appear to be important developmental periods, both for the process of socioeconomic stratification and for the onset of mental disorders. A classic early study of social class and mental disorder concluded that selection and drift, rather than social causation, were the dominant explanation for the relationship between socioeconomic status and mental disorder. In analyzing data on social class, ethnic group, and mental disorder, it is important to compare rates in ethnic groups at the same level of social class. Research on the mediating variables may eventually prove more helpful in improving the lives of individual human beings.