Hereditary Nonpolyposis Colorectal Cancer (HNPCC), also known as Lynch Syndrome, is an autosomal dominant, tumor predisposing disorder usuallycaused by germline mutations in mismatch repair (MMR) genes. A subset of HNPCC, Muir-Torre Syndrome (MTS) also involves MMR gene defects and is generally accepted as a variant of HNPCC. MTS is typicallycharacterized by at least one visceral malignancy and one cutaneous neoplasm of sebaceous differentiation, with or without keratoacanthomas. In either version of the disorder, nonfunctional MMR systems lead tothe loss of genomic integrity, marked commonly by mismatches in repetitive DNA sequences, resulting in microsatellite instabilities. Deleterious nucleotide alterations ultimately drive the process of tumorigenesis in both HNPCC and MTS. The following article reviews the epidemiology, genetics, clinical presentation, and management of HNPCC and its MTS variant.