A systematic approach to the evaluation and characterization of treatment resistance in schizophrenia has become increasingly important since the introduction of clozapine, risperidone, and olanzapine. The need for accurate evaluation will increase with the introduction of the next generation of antipsychotic medications. People with schizophrenia may manifest a poor response to therapy secondary to intolerance of medication, poor compliance, or inappropriate dosing, as well as true resistance of their illness to antipsychotic drug therapy. Clinicians facing the decision of when to change from one antipsychotic to another must clearly understand the appropriate length of a trial and what target symptoms respond to antipsychotics in order to maximize the response in patients with treatment-resistant schizophrenia.