Phospholipase 4 (P-)’ catalyzes the hydrolysis of the sn-2 fatty acyl bond of phospholipids to liberate free fatty acids and lysophospholipids (for review see Ref. 1). PLA, plays a central role in diverse cellular processes including phospholipid digestion and metabolism, host defense, and signal transduction (for review see Ref. 2). P 3 also provides precursors for eicosanoid generation when the cleaved fatty acid is arachidonic acid or for plateletactivating factor formation when the sn-1 position of the phospholipid contains an alkyl ether linkage (for review see Ref. 3). Pws are a diverse class of enzymes with regard to function, localization, regulation, mechanism, sequence, structure, and role of divalent metal ions. This review will describe several of the best studied Pws as a means of defining the various types of P-. The classification of PLA, s is discussed in the next section, followed by discussions of each type of enzyme and ending with a general discussion of PLA, function. While the human enzymes are naturally of greatest interest, the bulk of our knowledge of PLA, s has been obtained by studying enzymes from non-human sources. However, since most of the human Pws have essentially identical counterparts, the knowledge gained from the non-human enzymes is equally applicable to the human forms.