Radicalisation has become a highly influential idea in British policy making. It underpins and justifies Prevent, a core part of the UK's counter-terrorism strategy. Experts have theorised the radicalisation process, often beset by a weak evidence base and mired in fundamental contestation on definitions and explanatory factors. Experiential experts have been active contributors to these debates, presenting a challenge to the low-ranking role often given to experiential knowledge in evidence hierarchies and a contrast to policy areas in which it remains poorly valued. This paper draws on interviews with radicalisation experts to examine the dynamics of this pluralisation in practice. With a focus on credibility contests, it explains how experiential experts can claim authoritative knowledge and the challenges they face from those who prioritise theory-driven empirical data as the basis for contributions to knowledge. The paper draws out the implications for understandings of expertise of this newly conceptualised, evidence poor and highly applied topic area.