This article proposes the notion of 'evidence-based activism' to capture patients' and health activists' groups' focus on knowledge production and knowledge mobilisation in the governance of health issues. It introduces empirical data and analysis on groups active in four countries (France, Ireland, Portugal, and the UK), and in four condition areas (rare diseases, Alzheimer's disease, ADHD - Attention Deficit Hyperactivity Disorder, and childbirth). It shows how these groups engage with, and articulate a variety of credentialed knowledge and 'experiential knowledge' with a view to explore concerned people's situations, to make themselves part and parcel of the networks of expertise on their conditions in their national contexts, and to elaborate evidence on the issues they deem important to address both at an individual and a collective level. This article argues that in contrast to health movements which contest institutions from the outside, patients' and activists' groups which embrace 'evidence-based activism' work 'from within' to imagine new epistemic and political appraisal of their causes and conditions. 'Evidence-based activism' entails a collective inquiry associating patients/activists and specialists/professionals in the conjoint fabrics of scientific statements and political claims. From a conceptual standpoint, 'evidence-based activism' sheds light on the on-going co-production of matters of fact and matters of concern in contemporary technological democracies.