This work explores both the interactions, influences and negotiations which take place between expert and lay groups in the evolution of medical scientific knowledge, as exemplified in the case of repetitive strain injury RSI. Posing questions such as "how is medical knowledge developed?" and "what power structures are involved?", this study informs contemporary debates in the sociology of scientific knowledge and explores the practical implications of lay intervention, bridging sociology theory, medical science policy and activist concerns. Investigating the work related condition, RSI, the author draws upon evidence collected from a variety of stakeholders in the controversy, including GPs, sufferers, ergonomists and physiotherapists. This study should be of interest to students of sociology and related disciplines, as well as practitioners, activists, policy makers and general readers.