The supposed superiority of randomized over non-randomized studies is used to justify claims about therapeutic effectiveness of medical interventions and also inclusion criteria for many systematic reviews of therapeutic interventions. However, the view that randomized trials provide better evidence has been challenged by philosophers of science. In addition, empirical evidence for average differences between randomized trials and observational studies (which we would expect if one method were superior) has proven difficult to find. This chapter reviews the controversy surrounding the relative merits of randomized trials and observational studies. It is concluded that while (well-conducted) observational can often provide the same level of evidential support as randomized trials, merits of (well-conducted) randomized trials warrant claims about their superiority, especially where results from the two methods are contradictory.