This document discusses therapeutic drug monitoring (TDM), including its definition, introduction, criteria for when it is useful/unnecessary, and process. TDM involves measuring drug concentrations in blood/plasma to help adjust dosages to a desired therapeutic range. It is especially useful for drugs with a narrow therapeutic index or large interindividual variability. The TDM process involves collecting a biological sample at steady state, requesting a lab analysis, the lab measuring the drug level using an appropriate analytical technique, communicating the results along with the therapeutic range, and the clinician interpreting the level based on dosage and patient factors. Commonly monitored drugs and some problems with TDM services are also mentioned.