The Supreme Audit Institutions play an important accountability role by reporting to parliaments on the efficient, effective and cost-effective implementation of, amongst other things, environmental and energy policies. Climate change is considered by both United Nations (UN) and EU as one of the biggest environmental, economic and social challenges, and needs to be addressed in a coordinated effort at an international level. Emissions trading is a key policy instrument in meeting national and the Kyoto Protocol emissions targets in a cost-effective way. The implementation of the EU Emissions Trading System (EU ETS) and the project-based mechanisms under the Kyoto Protocol (the Clean Development Mechanism (CDM) and Joint Implementation (JI)) have been a huge administrative undertaking and entail new tasks and roles for governments and companies. There are potential risks related to the implementation of these systems as well as to their effectiveness. The aim of the cooperative audit has been to assess the trustworthiness, reliability and effectiveness of the EU ETS and project-based mechanisms under the Kyoto Protocol. This report draws on findings gained from individual audit reports from seven countries in the years 2008–2012.