BACKGROUND & AIMS: Somatic mosaicism, in which variants arise post-zygotically and are therefore not present in all cells in the body, may be an underestimated cause of colorectal cancer (CRC) and polyposis syndromes. We performed a systematic review to provide a comprehensive overview of somatic mosaicism in patients with CRC and polyposis syndromes. METHODS: We searched PubMed through March 2018 to identify reports of mosaicism in patients with CRC or polyposis syndromes. We divided the final set of studies into 3 subgroups describing APC mosaicism, mosaicism in other CRC susceptibility genes, and epigenetic mosaicism. RESULTS: Of the 232 articles identified in our systematic search, 46 met the criteria for further analysis. Of these, 35 studies described mosaic variants or epimutations in patients with CRC or polyposis syndromes. Nineteen studies described APC mosaicism, comprising a total of 57 patients. Six described mosaicism in genes associated with familial CRC syndromes, such as Lynch and Cowden syndromes. Ten studies described epigenetic mosaicism, sometimes resulting from a germline variant (such as deletion of EPCAM). CONCLUSIONS: We found that somatic mosaicism is underdiagnosed but critical for determining the clinical management of patients with de novo polyposis who possibly carry mosaic APC variants, and present a decision tree for the clinical management of these patients. Mosaicism in genes associated with susceptibility to CRC contributes to development of other familial CRC syndromes. Heritable epigenetic mosaicism is likely underestimated and could have a dominant pattern of inheritance. However, the inheritance of primary mosaic epimutations, without an underlying genetic cause, is complex and not fully understood.