Genetic epidemiology focuses on how genetic factors and their interactions with other risk factors increase vulnerability to, or protection against, disease. The investigations in genetic epidemiology are typically based on a combination of study designs including family, twin, and adoption studies. Migrant studies are perhaps the most powerful study design to identify environmental and cultural risk factors. This chapter presents a summary of relative risks derived from controlled family studies of selected psychiatric disorders such as schizophrenia, anxiety disorders, substance use disorders and mood disorders. Two factors which contribute to the complexity of the patterns of inheritance of psychiatric disorders are lack of validity of classification of psychiatric disorders and complexity of the pathways from genotypes to psychiatric phenotypes. The chapter reviews the role of the tools of epidemiology in ongoing and future studies designed to identify genes underlying mental disorders.