Cluster randomised trials (CRTs) are controlled trials in which the randomisation is applied to groups of individuals (clusters) as opposed to individual research participants. CRTs are increasingly being used in a wide variety of research with public health implications (including education). Well conducted CRTs can have a significant effect on policy-making. However, CRTs are only justified in specific circumstances where they might be considered suitable: this might be for reasons of scientific validity, feasibility, appropriateness, to reduce chances of contamination, etc.