The Rab proteins are small GTP-binding proteins and form the largest group within the Ras superfamily. Around 60 mammalian Rab proteins are known. Each Rab is localised to a specific subcellular compartment. Rabs are involved in vesicular traffic of proteins between all compartments of the cell during the exocytic and endocytic pathways. Rabs also interact with cytoskeletal elements and influence the movement of intracellular membrane vesicles and tubules. The Rab proteins are posttranslationally modified at their C-termini by addition of the 20-carbon isoprenoid, geranylgeranyl, mediated by Rab GGTase and this modification is crucial for their function. Mutations in genes encoding proteins essential for the prenylation reaction, Rab Escort Protein and Rab geranylgeranyl transferase, lead to genetic diseases called Choroideremia and Hermansky-Pudlak syndrome.