This chapter reviews intestinal wall structure and examines how mucosal functions are supported by the organization of the gut and the biological properties of the epithelial barrier and transepithelial transport. The epithelial barrier consists of numerous components that prevent injury from the varying potential insults. Some of these components are site specific and others are common throughout the gastrointestinal tract. These barriers may be conceptually divided into two major categories: those that are extrinsic to the epithelium and those provided by the physical presence of the epithelium, which are described as intrinsic barriers. Gut epithelial function may be modulated by a host of influences from nonepithelial sources, including growth factors, cytokines and chemokines, and neural mediators. Together, these signaling and regulatory networks interact to maintain gastrointestinal mucosal homeostasis. Maintenance of the epithelial barrier is critical to these functions and requires integrity of cellular plasma membranes and intercellular tight junctions.