Much is being learned about how plasma membrane composition and structure are regulated. The anatomy of vesicular trafficking has been described reasonably well, and the biochemical processes that achieve proper targeting of lipids and proteins are beginning to be elucidated. Information within the structures of proteins is being discovered that helps to target them to, or retain them in the endoplasmic reticulum, various portions of the Golgi, lysosomes, endosomes, or the plasma membrane. Moreover, investigators in this field are beginning to study how the cell deciphers this information. The mechanisms by which the cytoskeleton and its associated motor proteins help to move vesicles to the correct destinations are also being described, as are the coat, docking, and fusion proteins that result in formation, recognition, and fusion of vesicles with the correct target compartments. One of the most challenging problems in the field appears to be to describe the processes by which lipids and proteins are sorted to apical membranes of polarized cells, since sorting to this compartment appears to differ mechanistically from much of what is known about sorting and vesicular trafficking. In contrast to the emerging nature of knowledge concerning modulation of the function of membrane proteins by membrane lipids and the cytoskeleton, much is already known about the biochemical actions of the proteins. The action of biomembrane transport proteins depends on the formation of a barrier to the free mixing of intracellular and extracellular constituents by the membrane lipid bilayer. Moreover, transport across the membrane barrier frequently results in or requires work.