Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL-and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzymeʼs activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.-Zechner, R., P. C. Kienesberger, G. Haemmerle, R. Zimmermann, and A. Lass. Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores. J. Lipid Res. 2009. 50: 3-21.Supplementary key words lipolysis • hydrolase • neutral lipid storage disease Lipid homeostasis reflects a balance of processes, designed to generate fatty acids (FAs) and lipids, deliver them from their site of origin to target tissues, and catabolize them for metabolic purposes. Innumerable genes and signal components are responsible for an integrated communication network between many tissues and organs, including adipose tissue, liver, muscles, the digestive tract, pancreas, and the nervous system. This network ultimately accounts for the accurate regulation of lipid and energy homeostasis. Despite the central physiological importance of these processes for human health, many basic mechanisms regulating the synthesis, uptake, storage, and utilization of lipids remain insufficiently characterized.FAs are vital components of essentially all known organisms. They are important substrates for oxidation and the production of cellular energy. FAs are essential precursors for all lipid classes, including those forming biological membranes. Finally, they are important for protein function in acylated proteins and as ligands for nuclear receptor transcription factors. In contrast to these "beneficial" characteristics, unesterified FAs can become deleterious for cells when present even at relatively low concentrations. The chronic exposure of nona...