Epithelial cells line the lumens of organs and thus constitute the interface between the body's interior and exterior surfaces. This position endows these cells with the important task of regulating what enters and what is exported from the body. In order to accomplish this function, epithelia must have structurally and functionally distinct membrane surfaces: the apical surface exposed to the lumen, and the basolateral surface in contact with the laterally adjacent epithelial cells, and the connective tissue and capillary network below the epithelia. The specific lipid and protein contents of the apical and basolateral membrane surfaces are determined by a number of sorting and retention mechanisms. Many of these sorting and retention mechanisms are shared with other polarized cell types including neurons and certain cells of the immune system. This chapter focuses on recent advances in understanding how these various mechanisms facilitate the generation, maintenance, and dynamic regulation of protein and lipid trafficking within epithelial cells.