Certified emission reductions (CERs) from Clean Development Mechanism (CDM) projects have traditionally served as an indirect link between cap and trade systems around the world. However, since 2010, import restrictions have increased. Reasons for import limitations include the supplementarity principle, genuine concerns about the environmental integrity of CERs and social benefits of CDM projects, pressure from domestic emissions mitigation industries, concerns about competition in the industries in which reductions take place, as well as the attempt to pressure advanced developing countries to accept national emissions commitments under a future international climate policy regime. It is shown that import limitations lead to a decrease in CER prices and a race to generate CERs as quickly as possible. Such effects are visible in the CDM market after the EU announced its import limitations. The exclusion of CERs from specific project types will distort the CDM supply curve and increase the CER price unless the marginal abatement costs of the excluded project type are above the CER world market price. Similarly, exclusion of CERs from specific host countries will increase the price. Substantial differences are found in CER access to national carbon markets around the world.Policy relevanceCDM regulators could try to improve access of CERs to cap and trade schemes through improvements to additionality testing, standardizing baseline and monitoring methodologies and stakeholder consultation. However, regulators should be aware that standardization is no panacea, and controversies may resurface if standardized additionality determination (e.g. through benchmarks or positive lists) are applied for a certain period and found to be problematic. However, domestic policy concerns such as an unwillingness to send money abroad to buy credits, an inability to control market prices, and competitiveness impacts cannot be resolved by CDM reforms. If, despite such reforms of the CDM, blatant protectionism continues, a challenge before the World Trade Organisation (WTO) could be launched to stop discrimination of service exports from specific countries.