This article describes the judgements used to interpret evidence in evidence-based medicine (EBM) and health technology assessment (HTA). It outlines the methods and processes of EBM and HTA. Respectively, EBM and HTA are approaches to medical clinical decision making and efficient allocation of scarce health resources. At the heart of both is a concern to review and synthesise evidence, especially evidence derived from randomised controlled trials (RCTs) of clinical effectiveness. The driver of the approach of both is a desire to eliminate, or at least reduce, bias. The hierarchy of evidence, which is used as an indicator of the likelihood of bias, features heavily in the process and methods of EBM and HTA. The epistemological underpinnings of EBM and HTA are explored with particular reference to the distinction between rationalism and empiricism, developed by the philosopher David Hume and elaborated by Immanuel Kant in the Critique of Pure Reason. The importance of Humian and Kantian principles for understanding the projects of EBM and HTA is considered and the ways in which decisions are made in both, within a judgemental framework originally outlined by Kant, are explored.