Therapeutic drug monitoring can be defined as a multiple step activity involving: (i) the measurement of a drug; (ii) the interpretation of the concentration by taking into account the concentration-effect relationship; (iii) the individual dose adjustment. For over 15 years pharmacology-toxicology departments have implemented liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) on a daily basis. This article discusses the current position of this technology in TDM routine activity.