Tuberous sclerosis complex (TSC) results from a germline mutation in either the TSC1 or TSC2 gene. It has autosomal dominant inheritance with 95% penetrance. Sixty percent of TSC patients have no prior family history of TSC and represent new germline mutations. The manifestations of TSC include seizures, mental retardation, autism; benign tumors of the brain, retina, kidney, heart, and skin; and pulmonary lymphangiomyomatosis (LAM). TSC has a wide range of phenotypic variability and many of the phenotypes are age or gender dependent. For example, seizures and kidney tumors occur in approximately 80% of individuals with TSC, mental retardation in approximately 50%, and LAM in approximately 30% (women only). This phenotypic variation occurs even within families carrying the same mutation. This variability and the high new mutation rate contribute to diagnostic uncertainty, especially in mild cases. Clinical genetic diagnostic assays are now available, but because of the size and complexity of the TSC1 and TSC2 genes, the absence of a mutation does not exclude a TSC diagnosis. Rapid advances regarding the biologic and biochemical pathways in which the TSC proteins participate have led to widespread optimism that specific targeted therapy for TSC patients will soon be available.