© 2015 Elsevier Ltd. In recent years, democracies around the world have begun to turn increasingly to referendums in order to engage citizens and lessen the distance between government and the governed. There has been a diffusion of guidance on good practice on the regulation of referendums with proposals from the Venice Commission of the Council of Europe and the Institute for Democracy and Electoral Assistance (IDEA), among others. Referendum campaign regulations in particular have proven controversial in many countries with changes often emanating from court rulings (Spain, Ireland and Denmark). The development of theories about the origins and impact of campaign regulations (elections and referendums) has not kept pace with newly emerging practices. This is especially true in the area of referendum campaigns. As a result, there are opportunities for researchers to systematically examine referendums. The field offers increasing scope for researchers to make policy-relevant contributions but first, it is necessary to understand and systematize which campaign regulations are in place before we can understand and assess what impact they have and whether they are in fact operating in the ways assumed. This article proposes an index of campaign regulation and an initial empirical application which allows for systematic cross national comparison of referendum campaign regulations.