More than 20 different hereditary cancer syndromes have now been defined and attributed to specific germline mutations in various inherited cancer genes. Collectively, the syndromes affect about 1 percent of cancer patients. An individual who carries a mutant allele of an inherited cancer gene has a variable risk of cancer that is influenced by the particular mutation, other cellular genes, and dietary, lifestyle, and environmental factors. Though hereditary cancer syndromes are rare, their study has provided powerful insights into more common forms of cancer. Somatic mutations in sporadic cancers frequently alter the inherited cancer genes, and the functions of cell signaling pathways have been illuminated by study of the affected genes. Further investigation of inherited mutations that affect susceptibility to cancer will aid efforts to effectively prevent, detect, and treat the disease.