Glycerolipids, by far the largest group of plant lipids, are structurally based on glycerol. This group includes glycodiacylglycerols, phospholipids, and acylglycerols such as triacylglycerol. The properties of the triacylglycerols depend on the fatty acids esterified with each of the three hydroxyl groups of glycerol. This chapter focuses on the biosynthesis of triacylglycerols, especially in developing seeds. In all compartments of plant cells, an acyl-CoA(ACP):sn-glycerol-3-phosphate acyltranferase (GPAT) and an acyl-CoA(ACP):sn-1 -acylglycerol-3-phosphate acyltransferase (LPAAT ) catalyze the first two steps in the course of de novo biosynthesis of glycerolipids. So far, only one of these acyltransferases, namely the soluble GPAT of chloroplasts, has been purified to apparent homogeneity. The plastidial acyltransferases of higher plants unlike those of Euglena can use both acyl-coenzyme (CoA) and acyl-acyl carrier protein (ACP) thioesters as substrates for the biosynthesis of phosphatidic acid (PA). Mitochondria, like plastids, are capable of de novo biosynthesis of glycerolipids.