 Testing medical interventions in humans typically poses some risks to the participants, no matter how many laboratory and animal tests precede it. In this way, the process of collecting data through clinical trials to improve health and well-being inevitably exposes research participants to some risks for the benefit of future patients. This brings us to the central ethical challenge posed by clinical research: When is it ethically permissible to expose research participants to risks of harm for the benefit of others? The present entry focuses on this concern, and canvasses the most prominent attempts to address it. The present entry largely brackets the range of interesting and important ethical challenges that arise in the course of conducting clinical research: How should it be reviewed? Who may conduct it? What must potential participants understand to give valid consent? May it be conducted in countries that will not be able to afford the intervention being tested? Do investigators have any obligations to treat unrelated medical conditions in participants that they uncover during the course of their research?