Hereditary gastrointestinal polyposis syndromes account for around 1% of all colorectal cancers; most of them are associated with a broad spectrum of extracolonic tumors. The initial diagnosis is based on endoscopic findings and polyp histology. Molecular genetic screening is important for the delineation of conditions with a similar phenotype such as autosomal dominant familial adenomatous polyposis (FAP) and autosomal recessive MUTYH-associated polyposis (MAP). Identification of the germline mutation in an affected person is a prerequisite for the exact evaluation of the recurrence risk in relatives and the predictive testing of asymptomatic persons at risk. Beside cases with attenuated adenomatosis or few colorectal adenomas, diagnostic difficulties are common among the hamartomatous polyposes such as the juvenile polyposis syndrome due to their broad clinical overlap and uncertainties in histological assessment. Several poorly defined nonhereditary polyposis syndromes and those with an as yet unknown etiology exist including hyperplastic polyposis syndrome. Early detection and accurate classification are essential since effective methods for surveillance and treatment are available.