As familial cancer syndromes, the neurofibromatoses exhibit complex phenotypes, comprising a range of tumor and nontumor manifestations. Although the three recognized forms of neurofibromatosis (NF1, NF2, and schwannomatosis) all feature the development of nervous system tumors, their underlying genetic bases are clearly distinct. The most prominent common feature of all three is the appearance of Schwann cell–initiated tumorigenesis of the peripheral nervous system. Recent progress in delineating the molecular function of the NF1- and NF2-encoded proteins, together with the development and use of manipulable mouse models, has led to important advances in understanding the pathogenesis of many features of neurofibromatosis. An important outcome of the study of neurofibromatosis-associated tumorigenesis has been insight into the more general molecular and cellular bases of nervous system tumors.