The global response to the AIDS pandemic aims for universal access to treatment and for pursuing every possible avenue to prevention. Skeptics, doubting that the huge increases in current funding levels needed for universal treatment will ever happen, would scale back antiretroviral treatment in favor of more cost-effective preventive interventions. Economics, politics, and science figure in this debate. But there is also a question of ethical principle: Is there a moral imperative to emphasize treatment, even if emphasizing prevention would save more lives? The authors examine moral arguments that address this question, and come down on the side of saving the most lives via prevention.