This report was prepared to inform the Carbon Market Policy Dialogue (CMPD) between the European Commission, as the regulator of the EU Emissions Trading System, and the regulatory authorities for the emissions trading systems (ETSs) of California, Québec, China, New Zealand, and Switzerland. The report deals with the implications of linking emissions trading systems (ETSs) that differ from each other in the level of environmental ambition. The report provides a conceptual framework and summarizes the relevant scientific literature; it describes the current status of the six ETSs represented in the CMPD and, finally, it offers up a few ideas for discussion.