The 2010 WHO classification identifies a new group of pretumor lesions of the large bowel — serrated masses, which includes hyperplastic polyps (HPs), sessile serrated adenomas/polyps (SSA/P), and traditional serrated adenomas (TSA). Serrated masses are a heterogeneous group characterized by serrated gland lumens and, in most cases, without dysplasia. An enlarged proliferative zone, elongated crypts, and no cytological atypia in addition to a serrated lumen are typical of HPs. SSA/P is characterized by the migration of the proliferative zone to the crypt walls, giving rise to specific architectural disorders, such as expanded and horizontally growing basal gland segments along the lamina muscularis mucosae. TSA is typified by short ectopic crypts that cannot reach the lamina muscularis and by epithelial eosinophilic changes. SSA/P and TSA have peculiar molecular genetic profiles and proven malignant potential.