Pharmacokinetic monitoring is not routine in the treatment of psychiatric disorders, although subtherapeutic or toxic plasma concentrations of psychotropic agents can result from standard doses because of interindividual variability of drug metabolism. Therapeutic plasma concentrations have been established for several of the tricyclic antidepressants and for lithium, as well as for carbamazepine and valproic acid (valproate sodium). Despite difficulties in extrapolating from concentration-effect research, therapeutic concentrations have also been determined for some antipsychotic drugs, in particular haloperidol and clozapine.Clinicians can use therapeutic drug monitoring to optimise dosage decisions with psychotropic drugs, in order to maximise efficacy and prevent toxicity, especially when individuals are nonresponsive to treatment or vulnerable to adverse reactions with standard doses because age, disease states or drug interactions complicate therapy. Although evidence from controlled-outcome studies is unavailable, TDM-assisted psychiatric treatment is potentially useful and cost effective, particularly when applied by clinicians who are knowledgeable of pharmacokinetics and who are aware of the limitations of laboratory findings.