Pharmacokinetics (PK) is derived from two words; ‘pharmacon’ meaning ‘drug’ and ‘kinesis’ meaning movement. Drugs are transported across various biological membranes by passive diffusion, filtration, and specialized transport. Drug absorption is the transport of drug from site of administration to blood circulation. Metabolism results inactivation of drug during its passage from site of absorption to systemic circulation. Results in decreased bioavailability and therapeutic response. Bioavailability (BA) is the fraction/percentage of drug that reaches systemic circulation following administration by any route. From systemic circulation drug is distributed to different tissue compartments. In this process it crosses many barriers, then reaches the site of action. Factors determining distribution include lipid solubility, ionization, vascularity, and binding to plasma and cellular proteins. Biotransformation/metabolism is the chemical alteration of drug in living organism. Drug excretion, applied pharmacokinetics, therapeutic drug monitoring, fixed dose combination, and methods of prolonging drug action are the other topics which are also discussed thoroughly in this chapter.