The function of caveolae is hotly debated. It now seems clear that caveolae are stable membrane domains that are kept in place by the actin cytoskeleton. However, this stability can be perturbed, leading to caveolar internalization. Caveolae are important in the regulation of various signaling processes, such as nitric oxide activity, and in cholesterol efflux and cholesterol-ester uptake. Caveolin deficiency particularly affects the cardiovascular system and the lungs but, because the knockout mice are viable, none of the proposed functions appears to be essential. Rather than having a specific function, caveolae might be considered to be multifunctional organelles with a physiological role that varies depending on cell type and cellular needs.