Companies are increasingly choosing to voluntarily issue sustainability, or corporate social responsibility, reports as a supplement to their annual financial reports. Recent publications have also reported on the related phenomenon of sustainability report assurance. Few if any studies, however, have begun to examine the assurance of sustainability reports in any detail. The study reported in this paper contributes to increasing the understanding of sustainability report assurance. Specifically, the paper reports on an examination of the recent sustainability reporting practices of the Fortune 100 companies. The study identifies the companies that did and did not issue sustainability reports; those that were and were not accompanied by an independent assurance report; the providers of the assurance services; and the assurance standards followed. Conclusions and recommendations for future research are provided.