The dose-response relationships of a drug are quite different among subjects due to pharmacokinetic and/or pharmacodynamic variations. The quantitative estimation of pharmacokinetic and/or pharmacodynamic characteristics of an individual subject is required for designing the optimum individualized dosage regimen for the subject. Concentration controlled pharmacotherapy can be one approach to minimize the inappropriate dosing caused by the pharmacokinetic variation. This article introduces some basic pharmacokinetic/pharmacodynamic concepts to understand the dose-response relationship of a given drug and to design optimum pharmacotherapy in a given patient with the data derived from therapeutic drug monitoring and/or literatures.