Neurofibromatosis 1, neurofibromatosis 2, and tuberous sclerosis are a set of dominantly transmitted disorders that have in common the tendency towards formation of tumors of the nervous system and other tissues. The genes for neurofibromatosis 1, neurofibromatosis 2, and one of two forms of tuberous sclerosis have been identified and appear to act as tumor suppressor genes. Information is accumulating about pathogenesis that may eventually improve our ability to diagnose and treat these disorders.