Rules of Origin (RoOs) are used by importing Customs authorities in the international trading system to determine if a product is considered as sufficiently linked to the exporting country to count as originating there, in order apply preferential or MFN (Most Favoured Nation) rates of tariff to the goods, and to check for quota, anti-dumping and related compliance. The importance of RoOs is due to the fact that duties and restrictions in many cases depend upon the source of imports. Proving origin will be a far bigger issue than it is now for UK business exporting to the EU after Brexit. With all businesses likely to have to rethink their compliance with Rules of Origin post-Brexit, this paper, based on a study carried out with the support of the British Chambers of Commerce, outlines the current Certificates of Origin regime and the options for change after Brexit. Read Briefing Paper 15- CERTIFICATES AND RULES OF ORIGIN: THE EXPERIENCE OF UK FIRMS and the Online Appendix