Psychiatric epidemiology is considered one of the main scientific foundations of psychiatry. This article reviews methodological issues and major studies in psychiatric epidemiology. Methods such as measures of frequency and association are discussed. Selection, observation, and confounding, the main sources of bias in epidemiological studies, are described. Types of study designs, such as cross-sectional, cohort, and case-control, are detailed. A historical perspective of major population-based and clinical studies is presented. Major findings of the Epidemiologic Catchment Area Study and the National Comorbidity Survey are also included. Finally, future directions in the field are discussed.