Abstract: Since the pathways of glycerolipid biosynthesis were elucidated in the 1950's, considerable knowledge has been gained about the enzymes that catalyze the lipid biosynthetic reactions and the factors that regulate triacylglycerol biosynthesis. In the last few decades, in part due to advances in technology and the wide availability of nucleotide and amino acid sequences, we have made enormous strides in our understanding of these enzymes at the molecular level. In many cases, sequence information obtained from lipid biosynthetic enzymes of prokaryotes and yeast has provided the means to search the genomic and expressed sequence tag databases for mammalian homologs and most of the genes have now been identified. Surprisingly, multiple isoforms appear to catalyze the same chemical reactions, suggesting that each isoform may play a distinct functional role in the pathway of triacylglycerol and phospholipid biosynthesis. This review focuses on the de novo biosynthesis of triacylglycerol in eukaryotic cells, the isoenzymes that are involved, their subcellular locations, how they are regulated, and their putative individual roles in glycerolipid biosynthesis.