This report details a study on whether countries should use certified emission reductions – or CERs – to achieve post-2020 targets under the Paris Agreement. The Clean Development Mechanism is the world’s largest greenhouse gas crediting mechanism and will continue to issue CERs until 2020. Countries are considering using those CERs for targets after 2020, under both the Paris Agreement and the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). The study found that recognizing all CERs for post-2020 targets is unlikely to trigger significant emission reductions beyond those that would occur otherwise. Instead, policy-makers could prioritize or limit eligibility to certain CERs for post-2020 targets. That could mean limiting use to CERs from new projects that are more likely to be additional or from implemented projects that are at risk of discontinuing greenhouse gas abatement. The authors also suggest that using CERs for multi-year targets -- rather than single year targets -- could avoid higher cumulative GHG emissions in pre-target years. The report also recommends robust accounting to address the risk of double claiming CERs.