Abstract A hallmark of eukaryotic cells is their possession of a set of membrane-enclosed organelles. The traffic of proteins between different organelles of the secretory and endocytic pathways is mediated by membrane-bound containers, termed vesicles. Figure 1 illustrates the basic stages of transport. Vesicles form by budding off from a donor compartment. The vesicle is transported to the acceptor compartment where it docks onto the membrane. This is followed by the final stage of fusion which releases the vesicle contents into the acceptor compartment. A variety of experimental strategies have begun to identify components of the transport machinery, which appear to be conserved in all eukaryotic cells. Every transport event is thought to require representatives from a set of conserved protein families. One of these families is that of the Rab proteins, a group of small, monomeric GTP-binding proteins related to Ras. The Rab GTPases will be the focus of this chapter.