Medicinal/Pharmaceutical chemistry deals with the invention , desin, development and both pharmacological and analytical characterisation of drug substances. Medicinal chemists are indispensible within the preclinical stages of drug development, and again as pharmaceutical chemists in drug internal control . The chapter describes the stages of drug development (hit to steer to preclinical candidate to drug substance), followed by a summary of the phases of drug activity concerning a drug#s formulation, pharmacokinetics and pharmacodynamics. Finally, classification systems for marketed drug substances are presented, with a stress on the classification by molecular target.Medicinal Chemistry a peer-reviewed journal, aims to cover all the latest outstanding developments in medicinal chemistry and rational drug design. The journal publishes original research, full-length/mini-review articles and guest edited thematic issues covering recent research and developments in the field.