Avoiding double counting of emission reductions is a key policy concern to Parties to the United Nations Framework Convention on Climate Change (UNFCCC). Double counting of emission reductions can occur when a single greenhouse gas emission reduction or removal, achieved through a mechanism issuing units, is accounted more than once towards attaining mitigation pledges. We systematically assess how double counting can occur and how it could be addressed. We identify that double issuance – the issuance of two units for the same reductions – and double claiming – the accounting of the same reductions both in a greenhouse gas inventory and in units towards attaining a mitigation pledge – are the most important forms of double counting. They can occur not only directly, but in rather indirect ways which can be challenging to identify. Addressing double counting effectively requires international coordination in three areas: accounting of units, design of mechanisms that issue units, and consistent tracking and reporting on units. While international agreement on principles for accounting and mechanism design is crucial to preventing double counting, the governance arrangements for implementation and international oversight could vary. This article discusses options and makes recommendations for rules to address double counting in two distinct periods: through 2020 and post-2020 under a potential new international climate regime.