The Index of Evidence explores how established notions of fact and evidence are changing in the so-called post-truth era, when it seems we have more information at our fingertips than ever before, but also more misinformation, disinformation, lies and fakery. On the Internet, of course, hierarchies of knowledge are being levelled and top-down expertise is increasingly challenged. This erosion of trust in medical authority is leading to a crisis of evidence in the public sphere. What is its value when every proof can be disputed and countered? In medical disciplines, too, there are opposing views about what counts as evidence, and even what it looks like. Debates have emerged about how raw data is put to use, about the role of narrative, and how scientific expertise relates to that of patient experience.