A review of the available information about fatty acid binding to plasma albumin is presented. Albumin is composed of a single polypeptide chain, folded so as to form three or four spherical units. The strong fatty acid binding sites probably are located in crevices between these spherical regions. The anionic form of the fatty acid binds to albumin. Most of the binding energy comes from nonpolar interactions between the fatty acid hydrocarbon chain and uncharged amino acid side chains that line the binding sites. The binding sites are somewhat pliable, and their configuration can adapt to fit the incoming fatty acid. Stepwise association constants for binding to human albumin of fatty acids containing 6-18 carbon atoms are presented. These data indicate that each mole of fatty acid binds with a different affinity and that the association constants for multiple binding diminish sequentially, i.e., kappa 1 greater than kappa 2 greater than kappa 3 greater ... greater kappan. Because of uncertainties concerning fatty acid association in aqueous solutions, the constants for the 14-18 carbon acids probably are not definitive. In the usual physiological concentration range, free fatty acids do not displace appreciable amounts of a second organic compound from albumin. Sensitive spectrophotometric analyses revealed, however, that even small increases in free fatty acid concentration alter the molecular interaction between human albumin and another organic compound.