Most of the major psychiatric disorders have been analyzed at each of several different levels. For example, at the broadest level, epidemiological studies have served to establish the incidence of disorders like schizophrenia and major depression in a number of different populations. Family and twin studies have been important in determining the heritability of certain mental illnesses, and chromosomal and linkage analyses have identified a number of discrete loci that appear to be implicated in disease susceptibility or, even directly, in the pathogenesis of some disorders. In a few cases, specific genes have been found to be mutated or polymorphic and proteins they encode are currently being analyzed. This article reviews how these different levels contribute to our understanding of a number of psychiatric disorders, including drug addiction, which has been the focus of much of our own work.