Driven by the ongoing discussion of corporate responsibility, growing numbers of companies have been publishing what have become known as sustainability reports. These reports have in part been subject to voluntary external assurance. Although the percentage of assured reports is significant, the market for this kind of assurance is still in an early stage of development. In this context the present article will ascertain the theoretical determining factors in the demand for voluntary external assurance and subject these in relation to the markets in Germany, the Netherlands and Great Britain to empirical analysis. In the context of the development of hypotheses shaped by agency theory, the constructs of agency costs and signalling can be distinguished. Variables measuring these constructs will be examined. It can be shown that the type and scope of the reporting (i.e., the choice of the Global Reporting Initiative ‘application level’), the existence of a sustainability department, and the size of the company are associated with the demand for voluntary assurance. Additional control variables are examined. Here it appears that the demand for external assurance is highest in Great Britain. Finally, research opportunities in this field will be indicated and recent normative developments briefly sketched.