Synopsis. The evolution of internal membrane systems has introduced many additional control steps into pathways that, although basically similar in prokaryotic cells, are less sophisticated and leave many aspects up to chance. Temporal and spatial control of secre? tion, quality control of the proteins secreted or inserted into the plasma membrane, and exquisite control over the selective degradation of macromolecules, for example, are apparently indispensible requirements in multicellular organisms, but are relatively unimportant in bacteria. Although complicated in detail, most membrane traffic in the eukary? otic cell can be reduced to a few basic principles. Such a reductionist's view provides a conceptual framework that allows the reader to organize an otherwise overwhelming amount of data on cellular membrane architecture and dynamics.