Human genetic studies over the last thirty years have demonstrated that for many behavioral traits and most common axis I psychiatric disorders, genetic influences are on the order of 35 to 80 percent or greater (Plomin et al., 2001). These influences on common traits and disorders represent the effects of numerous genes, each conferring small contributions to the phenotype of the behavior in question. These genes may interact with each other in highly complex ways which vary over the course of development and can be modulated by environmental changes. Hence, though it is straightforward to demonstrate the presence and magnitude of genetic effects contributing to traits and disorders, it remains exceedingly difficult to demonstrate which gene and neurodevelopmental mechanisms are involved. Further complicating the matter is that the phenotypes of many common disorders which are known to have complex genetic influences may be mimicked by rare single-gene mutations (which may provide clues to the mechanisms or pathways of these disorders). The main goals of this chapter are to review known genetic mechanisms involved in the development of psychopathology, emphasizing early onset disorders, and to demonstrate how to integrate epidemiological, clinical, and family study data to describe the genetic structure of disorders and traits. A perusal of findings from genetic studies of selected autistic syndromes illustrates the complex principles of inheritance that underlie neurodevelopmental traits.