Enzymes are responsible for bringing about chemical reactions under the very mild conditions that are essential to the existence of biological organisms. A central feature of enzyme catalysis is the free energy of the reaction (ΔG), which describes the amount of energy released or taken up when the reaction occurs. This term will determine the direction in which the reaction will proceed and is not influenced by the enzyme. A second central feature of enzyme catalysis is the free energy of activation or ΔG‡ which describes the energy barrier over which the reactants must pass in their transformation to product. The higher the energy of activation, the slower the reaction. Enzymes exert their influence by reducing the free energy of activation to levels that allow the reaction to proceed at low temperature. Finally, the third central feature of enzyme catalysis is the ‘transition state’. This is the structure that exists at the highest point of activation energy and it is the most difficult structure to attain throughout the course of the reaction. Thus, enzymes often function by lowering the energy of the transition state, thereby lowering ΔG‡. The first step of enzyme catalysis is responsible for many of the most outstanding properties of biological systems and explains the mechanism of action of many pharmaceuticals and other bioreactive molecules such as poisons, herbicides, pesticides, etc. The enzymes bind the reactants and align them for subsequent chemical reactions. The act of binding a molecule to a protein requires correct and complementary location of various functional groups on the protein and reactant. This requirement produces one of the most distinctive properties of biology which is specificity. Many pharmaceuticals and other bioreactive molecules are designed to take advantage of this specificity and to influence individual enzyme-catalyzed reactions. The binding process also explains why biological reaction kinetics display saturation behavior. That is, there is an upper limit to the rate at which a reaction can occur. The saturation point is determined in part by the nature of the binding site on the enzyme. Overall, the biological need to exist under mild conditions of low temperature and neutral pH produces a requirement for enzyme catalysis. The enzymes in turn are responsible for many of the most distinctive properties of biological systems and for many approaches that are used in medicine.