There has been a growing interest in serum drug monitoring over the past decade since it has become obvious that drugs are more often over- or underadministered than was generally considered to be the case. For instance, a retrospective study covering several years found that phenytoin and theophylline concentrations were within the therapeutic range in only about 50 % of the cases studied. Retrospective and current studies showed a clinical need for the individualisation of dosage regimes based on drug monitoring, e.g. in the cases of theophylline, tobramycin, antiarrhythmics and some antiepileptics. This manuscript is a result of the penetrating discussions within the working group "Drug Monitoring" of the Deutsche Forschungsgemeinschaft. It appears in a series of publications of the Senate Commission for Clinical-Toxicological Analysis dealing with monitoring of those drugs for which the determination of the concentration in blood is of paramount importance for a state-of-the-art therapy. Profiles on other drugs, e.g. phenytoin, digoxin, methotrexate and cyclosporine will follow.