Rab proteins are best known for their essential roles in exocytic and endocytic membrane trafficking, which encompass the constitutive and regulated secretory routes, endocytosis via caveolae or clathrin-coated vesicles(CCVs), micropinocytosis and phagocytosis. They control anterograde and retrograde trafficking between compartments to coordinate cargo delivery and membrane recycling and also subcompartmentalize organelles by organizing specific membrane domains that function in trafficking of cargo to different destinations (colored lines on the poster denote such domains; the micrograph illustrates alternating Rab7 and Rab5 domains on dilated early endosomes)(Barbero et al., 2002; Vitale et al., 1998; Vonderheit and Helenius, 2005). In this way, Rab GTPases regulate plasma membrane delivery, organelle biogenesis and degradative pathways (lysosomal and autophagic). They also contribute to celltype-specific functions, such as regulated secretion (secretory granules/lysosomes in endocrine and exocrine cells), synaptic transmission [synaptic vesicles (SVs) in neurons] and phagocytosis (in macrophages and dendritic cells). In epithelia, Rab GTPases help generate polarity by regulating the trafficking of junctional proteins and integrins and by defining epithelial transport circuits to cilia (connecting with intraflagellar transport, IFT), the apical (AM) and basolateral (BM) membranes, and apical recycling endosomes (AREs). They thus play major regulatory roles maintaining compartment identity, regulating cargo delivery, controlling protein and lipid storage/degradation and modulating specialized trafficking functions.