Randomised controlled trials are the most rigorous means of assessing the outcomes of interventions. A control group enables the ‘signal’ of the intervention effect to be distinguished from the ‘noise’ of the other influences in the absence of intervention. Randomised allocation to intervention and control groups allow as fair a comparison as possible. However, most trials and systematic reviews are not as useful as they could be. Trials generally focus on estimating effects, often failing to explore the mechanisms through which these occur or how these interact with context to generate different outcomes in different settings or populations. Systematic reviews concentrate on pooling effect estimates from multiple trials from different contexts, as though there were one underlying effect that can be uncovered by pooling. They often, like most trials, fail to examine mechanisms and how these might interact with context to generate different outcomes in different settings and populations. These omissions hinder trials and systematic reviews in their role of providing useful evidence for understanding which interventions are likely to be the most promising candidates for transfer to other settings and with other populations.