Clinical trials are the cornerstone of evidence-based medicine. By directly comparing different interventions they produce evidence on their relative efficacy and effectiveness This, in turn, can inform secondary research and guideline development to facilitate evidence-based clinical decision making. However, the quality of evidence stemming from clinical trials is frequently poor. Here, the pathway of evidence from basic research to the generation of implementable clinical guidelines will be described. Relevant factors related to trial design and reporting, such as the choice of trial comparators, outcomes and outcome measures, will be described and their influence on evidence synthesis will be discussed. Finally, recommendations on how to improve trials in order to increase their usefulness for evidence generation will be given.