Pharmacodynamics is the science which deals with the physiological effects of drug on the body. A drug after reaching the blood circulation is transported to its target which may be a receptor or an enzyme which the drug agonizes or inhibits. This causes a response which can be measured and are known as biomarkers. Mathematical methods are then used to describe their temporal profile and further, to predict the effect at different doses, or help in design of further studies. This chapter deals with basic aspects of pharmacodynamics such as targets; types of actions; biomarkers; and a variety of mathematical models and methods.