Therapeutic drug monitoring of concentrations of drugs in body fluids, usually plasma, can be used during treatment and for diagnostic purposes. The selection of drugs for therapeutic drug monitoring is important as the concentrations of many drugs are not clearly related to their effects. For selected drugs therapeutic drug monitoring aims to enhance drug efficacy, reduce toxicity or assist with diagnosis. Despite its apparent advantages, it has inherent limitations. Some large hospitals have services which provide support with drug monitoring and interpretation of results. The monitoring of therapeutic drugs involves measuring drug concentrations in plasma, serum or blood. This information is used to individualize dosage so that drug concentrations can be maintained within a target range. Drug concentration at the site of action cannot be routinely measured, but the desired or adverse effects may correlate better with plasma or blood concentrations than they do with dose. For a few drugs, concentration measurements are a valuable surrogate of drug exposure, particularly if there is no simple or sensitive measure of effect.