The type of research known as cluster randomized trials raises ethical questions not readily answered within the standard understanding of research ethics. What distinguishes a CRT is that it randomizes at the level of social groups rather than at the level of individual research participants: in a CRT, the regimen under study might be assigned to a village, hospital, or school. The organizational schemes of CRTs raise an assortment of fundamental ethical problems. In certain CRTs, the question of whether it is possible to obtain individual informed consent is central. Also, if the intervention is offered at the level of a sizable social group, there is usually no realistic possibility for individuals to opt out of the trial. Other ethical questions raised by CRTs have to do with identifying the subjects of research, and indeed with whether “research” is occurring at all.