This chapter provides a summary of the landmark study Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE). Phase 1 of CATIE evaluated whether there are measurable differences in effectiveness between antipsychotics (risperidone vs. olanzapine vs. ziprasidone vs. quetiapine vs. perphenazine) in the treatment of patients with schizophrenia. Starting with that question, the chapter describes the fundamentals of the study, including funding, study location, inclusion and exclusion criteria, how many participants were included, research design, interventions, follow-up, endpoints, outcomes, as well as criticisms and limitations. The chapter briefly reviews other relevant studies and information, discusses implications, and concludes with a relevant clinical case.