Phospholipase D (PLD) catalyzes hydrolysis of phospholipids with production of phosphatidic acid, which often acts as secondary messenger of transduction of intracellular signals. This review summarizes data of leading laboratories on specific features of organization and regulation of PLD activity in plant and animal cells. The main structural domains of PLD (C2, PX, PH), the active site, and other functionally important parts of the enzyme are discussed. Regulatory mechanisms of PLD activity are characterized in detail. Studies associated with molecular design, analysis, and synthesis of new nontoxic substances capable of inhibiting different PLD isoenzymes in vivo are shown to be promising for biotechnology and medicine.