Desmoid tumors are biologically benign but sometimes locally aggressive neoplasms consisting of fibroblasts. They occur in two contexts: sporadically (in the general population), and in patients with familial adenomatous polyposis (FAP). Although the histology of desmoid tumors is similar in the sporadic and FAP settings, the genotype and phenotype are different. In this chapter the differences between the two types of desmoids are discussed.