Think tanks are omnipresent in the British media as sources of expertise for journalists and politicians; they themselves claim that their ideas form the basis of new policies and that they have influenced the thinking of leading decision-makers. This article aims to shed light on whether think tanks have played a role in the modernisation of the Conservative party under David Cameron between 2005 and 2010. The institutions focused on here are Policy Exchange, Reform, Centre for Social Justice, ResPublica, Politeia and Civitas. If these think tanks influenced the Conservatives' modernisation, which important impulses did they give and how? The article also addresses the more general question of why think tanks are worth academic scrutiny.