Before planning a new clinical trial, it is essential to systematically review existing studies on a given topic to identify gaps in knowledge and best design a new trial. Several studies show that systematic reviews are not used when planning clinical trials. Reasons include researchers' perception that they are experts in the field and familiar with all relevant research, lack of funding for staff, lack of time, ethics committee requirements and lack of awareness of systematic reviews. Failure to consider existing evidence can lead to unnecessary or poorly designed trials. This puts participants at risk, wastes valuable resources and reduces public confidence in science. The article provides REVEAL guidance for clinical researchers to identify prior evidence.