The Lynch syndromes, also referred to as hereditary nonpolyposis colorectal cancer, are reviewed, with particular attention given to their natural history, diagnosis, surveillance, and management. Lynch syndrome I is characterized by an autosomal dominantly inherited predisposition to early onset colorectal cancer, with proximal predominance and an excess of synchronous and metachronous colorectal cancers. Lynch syndrome II manifests all of the features of Lynch syndrome I but, in addition, shows an integral association with extracolonic cancers, particularly carcinomas of the endometrium and the ovary. There are no premonitory physical signs or biomarkers of genotypic risk. Therefore, the natural history features of the Lynch syndromes are crucial to their surveillance and management.