Cluster randomised trials are an increasingly important methodological tool in health research but they present challenges to the informed consent requirement. In the relatively limited literature on the ethics of cluster research there is not much clarity about the reasons for which seeking informed consent in cluster randomised trials may be morally challenging. In this paper, I distinguish between the cases where informed consent in cluster trials may be problematic due to the distinct features of ‘population-based’ interventions, which have not (...) been adequately discussed in the research ethics literature, and the cases where informed consent may be problematic for reasons that investigators also encounter in other research designs. I claim that informed consent requirements in cluster trials should be adjusted to the level of risk involved, arguing for a more comprehensive notion of research risk than that currently found in the research ethics guidelines, and the amount of freedom to be sacrificed in relation to a particular research aim. I conclude that these two factors are the most important to consider when assessing whether a cluster study should proceed when informed consent is infeasible or difficult to obtain. (shrink)