This chapter describes the principal membrane-enclosed compartments, or membrane-enclosed organelles, of eukaryotic cells and briefly consider their main functions. It discusses how the protein composition of the different compartments is set up and maintained. The chapter also describes how certain membrane-enclosed compartments in a eukaryotic cell communicate with one another by forming small, membrane-enclosed sacs, or vesicles. It also discusses how different membrane-enclosed organelles may have evolved. It explains how vesicles shuttle proteins and membranes between intracellular compartments, allowing cells to eat, drink, and secrete. The chapter considers how these transport vesicles are directed to their proper destination, be it an organelle of the endomembrane system or the plasma membrane. Protein transport from the endoplasmic reticulum to the Golgi apparatus and from the Golgi apparatus to other destinations is mediated by transport vesicles that continually bud off from one membrane and fuse with another, a process called vesicular transport.