Triacylglycerol (TAG) is the primary unit of energy storage in eukaryotic cells. TAG comprises more than 90% of the content of white adipocytes, and TAG synthesis is essential in enterocytes for the absorption of dietary lipids, in hepatocytes for the synthesis and transport of VLDL, and in mammary epithelial cells for the production of milk. TAG is critical for the formation of the water barrier of skin, it acts as a mechanical cushion within joints and around internal organs, and it also serves an important role in insulation. In virtually every type of cell, stored TAG sequesters essential fatty acids (FAs) and precursors of eicosanoids, as well as the DAG precursor of phospholipids. Storage of FAs in TAG protects cells from the potential detergent-like properties of FAs or their acyl-CoA derivatives, which may injure cellular membranes. In addition, recent studies suggest that cellular stores of TAG in lipid droplets release FAs that are channeled selectively to β-oxidation and that act as signals to influence the transcriptional control of gene expression. Additional studies suggest that the process of synthesizing or degrading TAG produces lipid intermediates like lysophosphatidic acid (LPA), phosphatidic acid (PA), and diacylglycerol (DAG) that may serve as activators or inhibitors of signaling pathways controlled by peroxisome proliferator-activated receptor-γ (PPARγ), the mammalian target of rapamycin (mTOR), or protein kinase C (PKC) isoforms. These signaling pathways may link excess intracellular TAG storage with insulin resistance. It is the goal of this review to provide an overview of TAG metabolism as a dynamic process that allows its lipid participants to play numerous inter-related roles within cells. Thus, we will focus on both the synthesis and degradation of TAG, the enzymes involved and their regulation, and the overall physiological regulation of these dynamic processes. In addition to the lipolytic enzymes, this review will also highlight coregulatory proteins that affect the activity of specific lipases. We have chosen to focus primarily on information reported during the past 10 years and recommend to readers several excellent reviews that present more detailed information from older studies on the synthetic pathway as a whole; 1 on GPAT; 2 on GPAT, AGPAT, and PAP; 3, 4 on DGAT; 5 on the lipolytic pathway and ATGLl; 6 on HSL; 7 and on MGL. 8