Caveolae are small vesicular invaginations of the cell membrane. It is within this organelle that cells perform transcytosis, potocytosis and signal transduction. These "little caves" are composed of a mixture of lipids and proteins unlike those found in the plasma membrane proper. The chief structural proteins of caveolae are caveolins. To date, three caveolins (Cav-1, -2 and -3) with unique tissue distributions have been identified. Caveolins form a scaffold onto which many signalling molecules can assemble, to generate pre-assembled signalling complexes. In addition to concentrating these signal transducers within a distinct region of the plasma membrane, caveolin binding may functionally regulate the activation state of caveolae-associated signalling molecules.