The movement of proteins between compartments of the exocytic and endocytic pathways of eukaryotic cells is mediated by carrier vesicles. They bud from a donor compartment and are targetted to and fuse with the acceptor compartment. GTPases, proteins which bind and hydrolyze GTP, play key roles in the regulation of this vesicular protein transport. Heterotrimeric and small GTPases are involved in this vesicular traffic.