Pharmacodynamics is the study of actions of drugs on the body. It is concerned about the mechanisms of action of the drug, i.e. to know what drugs do and how they do it. Drugs produce their effects by interacting with the physiological systems of organisms. Drugs merely modify the rate of functions of various systems. Stimulation, depression, irritation, replacement, anti-infective action, and modification of immune status are the different ways drug act. Most drugs produce their effects by binding to specific target proteins such as receptors, enzymes or ion channels. Clinical response to the increasing dose of drug is defined by shape of dose-response curve (DRC). Simultaneous administration of 2 or more drugs can result in additive, synergistic, antagonist effect. Drug interactions, adverse drug reactions, general principles of treatment of poisoning mnemonics (ABCDEFGHI), pharmacovigilance, new drug approval process and clinical trials, and new drug application are also discussed briefly throughout this chapter.