“Post-truth” was last year’s word of the year, according to Oxford Dictionaries, and it refers to the phenomenon of an idea’s emotional appeal having more influence than its basis in fact. How ideas acquire legitimacy and authority in medicine (or not) suggests some of the ways in which medicine mediates social and cultural transformations of evidence to facts to truth. In practice and research settings, clinicians interpret evidence in light of their beliefs about the legitimacy and authority of that evidence, and one truth worth knowing in health care is that experts don’t always agree. Disagreement about which criteria should be used to assess the merits of clinical advice or implement changes to standards of care, for example, can generate uncertainty, distrust, and confusion among professionals just as it can among members of the public. This issue of the AMA Journal of Ethics considers what this means for medicine and for patients.