The energy and information processing that takes place inside a cell consists of thousands of individual chemical reactions. For these reactions to take place in a physiologically useful fashion, they must occur at a rate that meets the cell’s needs, and they must display specificity; that is, a particular reactant should always yield a particular product. Side reactions leading to the formation of useless or hazardous by-products must be minimized. In this chapter, we consider the key properties of enzymes, with a special look at the energetics of enzyme-catalyzed reactions.