Most patients treated for an episode of unipolar or bipolar major depression are treatment resistant in the sense that the majority do not achieve full remission with the first somatic or psychosocial treatment they receive. Little attention has been given to formalizing criteria for evaluating the nature and extent of treatment resistance, even though determining the adequacy and outcome of prior treatment trials is key in clinical decision making about subsequent treatment. Furthermore, determining the adequacy of prior treatment is essential since substantial evidence indicates that large numbers of depressed patients are undertreated, resulting in prolonged episodes and the appearance of "pseudoresistance." Adequacy of antidepressant treatment trials should be defined in terms of thresholds for the dosage and duration of medication, adherence, and clinical outcome. The Antidepressant Treatment History Form is presented as one method to formalize the evaluation of treatment adequacy and treatment resistance.