Fatty acid binding proteins (FABPs) are proteins that reversibly bind fatty acids and other lipids. So far, nine tissue-specific cytoplasmic FABPs have been identified. Functions attributable to these lipid binding proteins remain unclear, but data are presented that indicate physiological roles in 1) fatty acid transport, esterification, and oxidation, 2) steroidogenesis, and 3) retinoid uptake, retinaldehyde reduction, and retinol esterification. Adipose tissue FABP (FABP4) binds both long chain fatty acids and retinoic acid. It delivers retinoic acid to nulear trans-acting proteins, thereby modulates genes coding for key proteins involved in lipid metabolism or differentiation. FABP4 is closely associated with metabolic syndrome, type 2 diabetes, and atheroscelerosis.