Many gastrointestinal polyposis syndromes have well-defined clinical and histopathologic features. Recent advances in our ability to analyze these syndromes at the genetic level have allowed for further characterization of these entities and their pathophysiology. Elucidation of the underlying molecular genetic alterations that result in the various forms of adenomatous polyposis syndromes has been reported recently and is discussed in this article. Defining these syndromes at the genetic level has important biologic and practical implications.