Physiologically, there is a balance of cell proliferation and exfoliation which maintains normal small-intestinal mucosal zonation with long villi and shallow crypts. In disease states and in the experimental animal this balance may be disturbed by numerous exogenous and endogenous factors, which may lead to three principally different responses: mucosal atrophy, mucosal hypertrophy, and mucosal transformation of the hyperregenerative type. The structural and functional characteristics of these different mucosal patterns are described, and their pathophysiologic relevance is discussed.