Therapeutic drug monitoring (TDM) among various fields of clinical pharmacology plays the vital role in order to achieve the indivisualization of optimal drug therapy, which is the process of maximizing therapeutic effects and minimizing side effecs, simultaneously. Variable drug responses are largely due to the individual pharmacokinetic variations. Therefore in TDM it is very important to keep optimal drug concentrations, which makes it possible to optimize the clinical outcomes of each patient. In recent, the introduction of clinical pharmacokinetic principles and development of assay methods makes TDM more prevalent. This article describes the clinical significance, functions, and applications of TDM in several aspects. TDM indications, concepts and examples of therapeutic range are also described. The features which should be strived for in each step of TDM process (the decision to request a drug level, blood sampling, laboratory measurement, clinical interpretation of drug concentration, and production of individualized dosage regimen and dosage adjustment) are discussed.