Background: This annotation describes the uses of randomised controlled trials (RCTs) in clinical child psychology and psychiatry. Method: It explores the scientific basis for randomised designs, the conceptual and methodological issues that can arise when using them, alternative methods, and future directions. Results: There are many issues that have to be tackled when using randomised trials to answer questions about the effectiveness of interventions used by child mental health professionals. The most important are conceptual issues concerning the design of these studies, practical issues, and issues about the interpretation of the results. There are some situations in which randomised trials are not possible or ideal and alternative strategies may therefore be needed. Future RCTs should be more explicit about whether their primary purpose is to further scientific knowledge or to evaluate the benefit of a treatment in routine clinical practice. Future trials should also have outcomes of unequivocal significance and be reported in accordance with standardised guidelines. Conclusions: Well-designed and unambiguously reported RCTs usually provide the best possible evidence about the effectiveness of an intervention. RCTs are not, however, the only way of establishing cause and effect and their results should always be interpreted in the light of other evidence.