Many think-tanks are said to be influential but what is meant by influence is invariably imprecise. Conceptual devices such as policy or epistemic communities explain the routes of influence but cannot quantify the impact of think-tanks. It is the aim of the first section of this chapter to explore some further views of think-tank influence, effectiveness and relevance. Too often, discussions of think-tank influence have concentrated solely on political or policy impact to the neglect of other patterns of influence. The following section outlines the way in which policy research institutes establish an identity as independent and educational organisations committed to promoting the public interest. Establishing this identity is important in order to be effective, that is, to be regarded as a reputable and authoritative source of opinion as well as to retain their concessionary tax status as non-profit organisations.