Drawing on fieldwork in four condition areas (rare diseases, childbirth, Attention Deficit Hyperactivity Disorder, and Alzheimer's disease), this article shows that patients' organizations' (POs) engagement with knowledge is neither limited to a set of diseases nor restricted to biomedical knowledge. Their work on and with academic and experiential knowledge contributes to an understanding of their conditions and the problems they induce, and to the shaping of the causes they defend. This results in the production of new evidence for grounding research and health policies in their condition areas. The authors propose the notion of "evidence-­‐based activism" to capture the centrality of knowledge activities in contemporary POs.