Can be defined as the study of the physical and chemical properties of drugs and their proper dosage as it relate to the onset, duration, and intensity of drug action, or it can be defined as the study of the effects of physicochemical properties of the drug and the drug product. examines the interrelationship of the physical/chemical properties of the drug the dosage form (drug product) in which the drug is given and the route of administration on the rate and extent of systemic drug absorption. It is based on fundamental scientific principles and experimental methodology. Use both In-vitro & In-vivo methods