Phosphatidate lies at an important branch point in metabolism in mammalian tissues. It can be converted to CDP-diacylglycerol which acts as a precursor for the synthesis of acidic phospholipids such as phosphatidylinositol, phosphatidylglycerol, and diphosphatidylglycerol (Figure 1). Alternatively, the phosphatidate can be converted to diacylglycerol through the action of phosphatidate phosphohydrolase (EC which is the subject of this book. Diacylglycerol can then serve as a substrate for the synthesis of the zwitterionic phospholipids, phosphatidylcholine and phosphatidylethanolamine, or it can be converted into triacylglycerol. In plants diacylglycerol is also converted into galactolipids (Chapter 4, Figure 1). Chapter 4 also describes the role of phosphatidate phosphohydrolase activity in microorganisms. A further route of metabolism for phosphatidate in mammalian liver is its deacylation back to glycerol phosphate by phospholipase A type activities (Chapter 2, Sections II and V). This reaction forms part of a substrate cycle that can decrease the accumulation of phosphatidate in membranes and prevent it from being incorporated into glycerolipids.