 One might attempt to address the central ethical challenge by limiting clinical research to the medical setting, offering experimental interventions to patients who want to try them. This approach, which has the virtue of evaluating interventions in the process of trying to help individual patients, makes sense for comparisons of two or more interventions that are widely accepted and already in clinical use. In contrast, this approach poses enormous scientific and practical problems with respect to testing new interventions. On the practical side, who would be willing to manufacture a new intervention without knowing whether it works? What dose should be used? How often should the new drug be taken? More importantly, this approach might not yield reliable information as to whether the new treatment is useful or harmful until hundreds, perhaps thousands of people have received it. Clinical research is designed to address these concerns by systematically assessing potential new treatments in a small number of individuals, including very sick ones, before offering them more widely. As we go about our daily lives, driving cars, flushing our waste down the drain, exhaling, getting a dog, we inevitably expose others to risks of harm. Despite the fact that these practices pervade our lives, there has been surprisingly little philosophical analysis of the conditions under which they are acceptable (Hayenhjelm and Wolff 2012). In addition to being of value in its own right, evaluation of the central ethical challenge posed by clinical research thus provides an opportunity to consider one of the more fundamental concerns in moral theory: when is it acceptable to expose some individuals to risks of harm?