The lipases and phospholipases represent a diverse group of enzymes that are expressed in animals, plants, fungi, and bacteria. Their ubiquitous distribution among all species is a testament to the essential roles played by these enzymes in lipid storage, mobilization and metabolism, membrane homeostasis and remodeling, endocrine and immune functions, and signal tra- duction. In humans, lipases and phospholipases are also thought to contribute to complex diseases, such as atherosclerosis, obesity, arthritis, and cancer, as well as to single gene defects, such as Wolman's disease and Type I hyperlipoproteinemia. Enzymatically, the lipases are unique, since they hydrolyze substrates that are either insoluble, or only partly soluble, in aq- ous solvents; thus, enzyme catalysis takes place at a lipid-water interface. The interface comprises at least two, and often more, discrete bulk and s- face phases, in which the enzyme, substrate, and products oflipolysis disperse among these phases based on their physical properties. Furthermore, the d- tribution of these components changes continuously as lipolysis proceeds. Thus, the lipases and phospholipases are fundamentally different from any other enzyme because of the physical complexity of the environment in which catalysis occurs.