Cancer, the uncontrolled proliferation of a population of somatic cells, is fundamentally a genetic disorder. Although the specific array of genetic changes causing individual tumor types remains largely obscure, the past two decades have witnessed a tremendous increase in our understanding of the specific genes regulating cell differentiation, proliferation, and senescence. There appear to be two distinct fundamental genetic mechanisms of tumorigenesis. One mechanism is associated with the activation of growth-promoting factors such as proto-oncogenes. Alternatively, tumor formation may be induced as the result of the loss or inactivation of genes which normally regulate or suppress cell growth. These genes have been termed 'tumor suppressor' genes or 'anti-oncogenes'. This review focuses on the role of 'tumor suppressor' genes in tumor formation and growth control of the human nervous system.