Medicinal chemistry is a discipline of pharmacy at the intersection of chemistry, especially synthetic organic chemistry, and pharmacology and various other biological specialties. It deals with the discovery, design, development and both pharmacological and analytical characterization of drug substances. Medicinal chemists are indispensable in the preclinical stages of drug development, and again as pharmaceutical chemists in drug quality control. The chapter describes stages of drug development (hit to lead to preclinical candidate to drug substance), followed by a summary of the phases of drug activity relating to a drug's formulation, pharmacokinetics and pharmacodynamics.