Medical researchers’ ancillary-care obligations have, until recently, been ignored by the authoritative guidelines on the ethics of medical research. Ancillary care is medical care, often unrelated to what is under study, that is not required by sound science, safe trial conduct, morally optional promises, or redressing research injuries. The question is when medical researchers have moral responsibilities to provide such care if their study participants need it. This question shows up insistently in studies done in resource-poor areas and—as the question of whether to return incidental findings—in genomic and imaging studies. After laying out six desiderata for a fully adequate account of medical researchers’ ancillary-care obligations, this chapter critically evaluates six potential grounds for such obligations—the duty of rescue, human rights, rectificatory justice, professional-role obligations, the researcher–participant relationship, and partial entrustment. It closes by suggesting the possibility of combining two or more of these grounds.