This report covers the work of the Executive Board of the clean development mechanism (CDM) during the period from 25 October 2008 to 16 October 2009, during which the CDM continued to grow steadily. More than 335 million certified emission reductions (CERs) have been issued to 576 registered project activities since the inception of the CDM. Over the reporting period, the Board considered 718 requests for registration and 510 requests for issuance, resulting in 644 additional CDM project activities and issuance of over 115 million CERs. There are now more than 5,000 CDM project activities (registered projects and those seeking registration). Were they all to deliver their estimated emission reductions, about 2.7 billion CERs would be generated during the first commitment period of the Kyoto Protocol. The report highlights achievements and challenges faced by the Board in its supervision of the mechanism. It highlights work undertaken in the areas of accreditation, methodologies and registration and issuance. It includes a number of recommendations for action by the Conference of the Parties serving as the meeting of the Parties to the Kyoto Protocol.