It is estimated that approximately 12% of the individuals under the age of 18 in the United States have a diagnosable mental illness [Institute of Medicine, 1989]; however, only a minority of the etiological research in psychopathology focuses on disorders with childhood onset. The present report demonstrates the usefulness of twin studies in exploring the etiology of childhood and adolescent psychiatric psychopathology and reviews the design, methodology, and results from traditional twin studies of various behavioral disorders. Alternative twin designs are also reviewed in an effort to address the future direction of twin studies in the area of childhood and adolescent psychopathology and to illustrate that twin data have much more to offer the field of psychopathology than merely an initial test to rule in or to rule out a significant genetic contribution to the development of such behaviors.