A variety of disciplines are involved in understanding the events that take place during the process by which a chemical entity (substance) becomes an active drug or a therapeutic agent. Principles of physics, physical chemistry, and mathematics are essential in the formulation of an optimum dosage form. An understanding of physiology and pharmacology is also essential in the process of screening for active drug and in selecting an appropriate route of administration. Equally important, knowledge of the principles of kinetics (rate processes), analytical chemistry and therapeutics is essential in providing an effective concentration of a drug at the ‘‘site of action.’’ Pharmacokinetics and biopharmaceutics are the result of such a successful integration of these various disciplines.