Active control noninferiority trials are being used with increasing frequency in new drug or device development when standard placebo-controlled trials are considered unethical. Nevertheless, the design and analysis of these trials are founded on a number of assumptions and arbitrary criteria that are generally not well understood or justifiable. Trials designed to show noninferiority require an appropriate reference population, a proven active control and dose, an appropriate margin of noninferiority that is clinically relevant and statistically justifiable, a high level of adherence to treatment, and adequate statistical power to reliably conclude that a treatment is truly noninferior and therefore effective. Accordingly, if noninferiority trials are to be applied to clinical and regulatory decisions regarding the marketing and use of new treatments, the assumptions must be made explicit and their influence on the resultant conclusions must be assessed rigorously. When conservative criteria were applied to each of the key assumptions underlying 2 representative noninferiority trials, they materially undermined the conclusions regarding noninferiority failing to confirm reported conclusions regarding noninferiority despite enthusiastic dissemination and acceptance of the results. Because the clinical, regulatory, and economic impact of active control noninferiority trials is substantial, robust criteria should be used routinely in their design, analysis, and interpretation to reach their intended objectives and to keep them from becoming wasted efforts.