ALTHOUGHTWIN TWIN studies have made a major contribution to the field of psychiatry over the last 65 years, recent developments have considerably expanded the range and sophistication of psychiatric twin research. Particularly important have been (1) the introduction of biometrical model fitting, (2) the use of population-based twin samples, and (3) the expansion of twin studies to include two or more disorders, specified environmental risk factors, multiple assessments or informants, nontwin relatives, and longitudinal observations. The equal environment assumption for psychiatric twin studies has been more rigorously examined and continues to receive empirical support. Twin studies are beginning to examine genotypeenvironment interaction in the etiology of psychopathologic conditions and to address the nature of putative environmental risk factors, such as stressful life events. Methods are being developed to incorporate specified genotypes, assessed using modem gene mapping techniques, into twin studies. With increasing conceptual rigor and methodologic sophistication, twin studies will