Enzymes are defined as the chemical reactions in all cells of living things operate in the presence of biological catalysts. An enzyme is a protein formed by the body that acts as a catalyst to cause a certain desired reaction. Enzymes are very specific. Each enzyme is designed to initiate a specific response with a specific result. This chapter covers the characteristics of enzymes, classifications of enzymes, functions of enzymes, mechanism of enzyme activity, factors affecting enzymes activity, isozymes, coenzymes, clinical important enzymes, and enzymes and disease conditions. Enzymes are biological compounds that increase the rate of a chemical reaction within the body. Almost all enzymes in the human body are proteins. All enzymes exhibit maximum activity under optimal conditions. Enzyme activity is markedly, affected by several factors. The important factors that influence the velocity of enzyme reaction are concentration of enzyme, concentration of substrate, effect of temperature, effect of pH, effect of product concentration, effect of activators, effect of time, and effect of light and radiation.