Epithelium forms continuous layers of cells that cover surfaces and line cavities of the body. These cavities include the closed peritoneal, pleural, and pericardial cavities, where the epithelium is called mesothelium, and open organ cavities, i.e. digestive, respiratory, and urogenital organs, which connect with the outside. In addition, epithelium lines the cardiovascular and lymph passageways as endothelium.

The parenchymal (secretory) cells of glands (e.g. sweat, salivary) are also epithelium. Epithelial cells are always in close apposition to each other, with a space between membranes of only about 20nm. A small amount of intercellular material, called cement substance (glycosaminoglycan) allows cells to glide over each other and offers only minimal resistance to the migration of leukocytes and other connective tissue cells through the epithelial layers. Epithelial attachment points (junctional complexes) occur between neighboring epithelial cells to hold adjacent cell membranes in close apposition. They also serve as anchoring sites for the fine filaments of the cytoskeleton, which assists in stabilizing the cell shape.

Epithelial cells rest on a basal lamina separating them from underlying connective tissue (CT). Epithelium is avascular and for its nutrition depends on diffusion of substances across the basement membrane.

The lumen is lined by an epithelium (stratified epithelium) epithelium rests on a vascular connective tissue, the lamina propria.