In this chapter, we have reviewed the behavior genetic literature for a number of the major forms of child and adult psychopathology, including schizophrenia, schizophrenia spectrum disorders, and schizotypal personality; affective and anxiety disorders in both children and adults; and the childhood disruptive disorders ADHD, ODD, and CD. Unfortunately, space limitations precluded our review of behavior genetic research on other important disorders, such as autism, substance abuse and dependence, and personality disorders. We also provided background material on the statistical methods used in contemporary twin studies to conduct univariate and multivariate behavior genetic analyses, as well as a brief discussion of some exciting new and future directions in behavior genetic studies of psychopathology. The latter included genetic and environmental influences on normal range variation and on the disordered extreme, the inclusion of specific environmental and endophenotypic variables in behavior genetic models, molecular genetic studies of psychopathology, and the examination of genotype-environment interaction and correlation. These future directions will facilitate the progression of behavior genetic studies of psychopathology beyond the simple estimation of broad, abstract genetic and environmental variance components for a single disorder to the examination of more complex models for the specific etiological mechanisms that underlie disorders and their overlap.