Therapeutic drug monitoring (TDM) is a branch of clinical chemistry that specializes in the measurement of medication levels in blood. It is also part of chemical pharmacology. Its main focus is on drugs with a narrow therapeutic index, i.e. drugs that can easily be under or overdosed. TDM is defined as “individualization of dosage by maintaining plasma or blood concentrations within a target range (therapeutic range/therapeutic window). It is pragmatic manipulation of the dose of a drug using plasma concentration as a guide to optimize its efficacy, to avoid or identify toxicity and to detect or confirm poor compliance”. Time of sampling, drug estimation methods, interpretation of plasma concentration data, and dosage adjustment are important steps in TDM. A detailed discussion on the TDM procedure at a glance has been given in this chapter.