Psychiatry has always struggled with the relative importance of nature versus nurture (McGuffin and Southwick 2003). In recent decades, advances in neuroscience have transformed debate over whether mental illness is biological or behavioral to an understanding that all behavior is biological and that this biology arises from genes, environments, and their interactions. Basic concepts in psychiatric genetics inform our current understanding of the biological basis of behavioral disorders and ongoing efforts to identify improved approaches to treatment. A glossary of common genetics terms is provided in Table 38–1.