The purpose of this paper is to review the literature on water and electrolyte movement across the intestinal mucosa in conceptual terms so that an understanding of the mechanisms of these movements and interrelationships will be apparent. We have not attempted to mention every or even most of the large number of recent articles that have reported data on water and electrolyte absorption. Rather, we have concentrated on articles which clarify the properties of the intestinal mucosal barrier with respect to water and electrolyte movement, and those which establish the mechanisms of transport across this barrier. Experiments on intestinal absorption, of necessity, must be interpreted as if the observed events are the result of the activity of a homogeneous group of cells all participating equally in a given transport process. In point of fact, however, the intestinal mucosa is a heterogeneous collection of cells; therefore, it is possible that the result of any experiment represents the net result of the different activities of the various cell types.