A key principle of evidence-based medicine is the recognition that not all evidence is similarly protected against error, and that decisions that rely on evidence would be more confident when the evidence is more protected against bias by virtue of the methods used. Thus, a fundamental principle of evidence-based medicine is the recognition of a hierarchy of evidence. A review of the different approaches the scientific method has evolved to protect evidence from bias is presented as well as the evolution of how methodologists have built hierarchies of evidence and note the limitations and merits of these approaches.