Cancer genetics is increasingly becoming integrated into the practice of modern medical oncology. The ability to distinguish a growing proportion of the 5% to 10% of all cancers that develop in individuals who have inherited a genetic mutation conferring heightened susceptibility to specific cancers may permit targeted efforts in cancer surveillance and prevention. While these individuals comprise a small proportion of the overall burden of cancer, strategies successful in reducing their remarkable cancer risks may be generalizable to the broader population. In this review, we highlight the most common hereditary cancer syndromes, most attributable to genes inherited in an autosomal dominant manner with incomplete penetrance, and a number of rare syndromes in which particular progress has been made. The prevalence, penetrance, tumor spectrum, and underlying genetic defects are discussed and summarized in a large table in which a more comprehensive enumeration of syndromes is provided.