Genetic background is suspected in about 20% of colorectal cancer (CRC) cases, in which either genetic polymorphisms or Mendelian heritable factors are involved. Currently known CRC syndromes include various polyposis syndromes (<1% of total CRC cases) and Lynch syndrome (LS), previously termed hereditary nonpolyposis colorectal cancer (HNPCC, comprises 3-5% of all CRC cases). LS is caused by dominantly inherited mutations in the mismatch repair genes MLH1, MSH2, MSH6 or PMS2, and results in a very high lifetime risk (approximately 80%) for CRC and significantly increased risk for extracolonic tumors in regions such as the endometrium, ovary, urinary tract, lymphoma, stomach, pancreas small bowel and brain. Carriers are advised to undergo specific medical and intense endoscopic surveillance. Diagnosis of carriers is mandatory for providing appropriate recommendations for surveillance, which was shown to decrease morbidity, mortality and health costs. Diagnosis of LS dictates preventive surgical procedures for the colon endometrium and ovaries, and assists in decisions regarding CRC chemotherapy. Family members' screening and surveillance is determined by mutation testing. Diagnosis is performed, based on the clinical selection criteria of Amsterdam and Bethesda and according to typical histology of tumor tissue. Initially, tumor testing is performed by either microsatellite instability (MSI), immunohistochemistry (IHC) or both. Certain Jewish ethnical subgroups may undergo founder mutation testing. Ultimate identification of the mutation by sequencing and MLPA is performed according to the IHC results. In families with hereditary CRC criteria, in which workup for LS is negative, the surveillance protocol will be determined by an experienced multidisciplinary team, including a formal genetic consultation.