Therapeutic drug monitoring (TDM) represents an early approach to personalised medicine. It helps the clinician to individualise drug treatment and guide dosage to reach systemic drug concentrations associated with therapeutic efficacy and/or to reduce the risk of concentration-dependent adverse effects. Well into the fifth decade of TDM as a service to healthcare, this concept is still expanding, and new areas for clinical implementation continue to emerge. The aim of this overview is to discuss promising new therapeutic areas in future TDM services, how to improve the clinical interpretation of single drug measurements and how recent technology development opens the doors to research and new applications.