This chapter discusses the molecular physiology of citrate. It discusses the properties of the enzymes that metabolized citrate. High concentrations of citrate are found in bone and other biominerals, in prostate and seminal fluid as a fermentation product of molds, and in citrus fruits. Citric acid has pK values of about 3.1,4.8, and 6.4 so that in most cells it will exist mainly as the trianion. The acid does not have an unpleasant sour taste and this property makes it commercially important as an acidulant in many pharmaceutical and food products. There are three kinds of proteins that bind citrate. There are the enzymes that form, break down, or modify citrate. There are proteins that transport citrate across biomembranes and, moreover, there are enzymes whose activities are affected when they bind citrate.