Radicalisation theory has emerged as the dominant explanatory framework for terrorism and underpins contemporary preventive counter-terrorism policy efforts in the UK. Its literature suggests, however, a conceptually, empirically and theoretically weak evidence base, underpinned by fundamental uncertainties associated with modelling and anticipating terrorism and embedded with controversy over interpretations of the available evidence. This thesis examines how credible expertise is established in this context. Drawing on data generated from interviews with radicalisation experts and document analysis, it first examines conceptualisations of radicalisation expertise. It identifies and explains the markers that are used to delineate expertise on the topic, including the role played by experiential knowledge. The second focus is on the constitution of the expert community. The analysis draws out the diverse forms of knowledge represented and traces the contributions of experts to public debates on the issue. The thesis finds that radicalisation is a topic area characterised by diverse forms of authoritative knowledge and conflicting perspectives as to what constitutes a ‘radicalisation expert’. This contestation within the expert community emerges in diverse public commentary on radicalisation, and the thesis identifies the presence of four expert ‘perspectives’ that are articulated in policy debates. This empirical account of the radicalisation expert community is considered in terms of the wider dynamics of contemporary expertise and the struggles inherent in identifying experts in a context of diverse knowledge claims.