Fharmacokinetic/pharmacodynamic (PK/PD) modeling links dose-concentration relationships (PK) and concentration-effect relationships (PD), thereby facilitating the description and prediction of the time course of drug effects resulting from a certain dosing regimen. This review gives some background information on PK/PD modeling and what non-linear mixed effects models are, why they can be useful in analysing repeated measures and what makes analysing such data challenging. NONMEM perform this kind of analysis, and will be reviewed briefly in this paper. In all phases of preclinical and clinical drug development, application of PK/PD modeling concepts are potentially beneficial. And broader application of PK/PD concepts in clinical therapy will provide a more rational basis for personalized medicine.