Enzymes are highly specialized proteins that can act as catalyst in biological systems. Enzymes can be of two types which are a simple enzyme and complex enzyme. Thermostable, low molecular weight, and nonprotein organic substances are called coenzymes. A coenzyme can bind covalently or noncovalently to the enzymes. As per IUBMB, enzymes are classified into six main classes which are oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases which are also explained briefly throughout this chapter. This chapter also discusses the mechanism of enzyme action, factors affecting enzyme activity, enzyme inhibition, regulation of enzymes, isoenzymes, enzyme profile for liver diseases, enzyme profile in prostate cancer, enzyme profile pancreatitis, serine proteases, and enzymes in other body fluids. At the end of the chapter, multiple choice questions are also given along with their explanatory answers.