The 21st Annual Meeting of the Greater Mekong Subregion (GMS) Working Group on Environment (WGE AM21), held on 23 June 2015 in Bangkok, Thailand, was chaired by Dr. Wijarn Simachaya, Deputy Permanent Secretary, Ministry of Natural Resources and Environment (MNRE), Thailand (morning, first half), Dr. Prasert Sornsathapornkul, Director, National Park and Protected Area Innovations Institute, Department of National Parks, Wildlife and Plant Conservation, MNRE (morning, second half), and Dr. Nawarat Krairapanond, Director, Natural Resources and Environmental Management Coordination Division, Natural Resources and Environmental Policy and Planning, MNRE (afternoon); and co-chaired by Mr. Pavit Ramachandran, Senior Environment Specialist, Asian Development Bank (ADB). The meeting was attended by representatives from the environment and other relevant ministries of Cambodia, the People’s Republic of China (PRC), the Lao People’s Democratic Republic (Lao PDR), Myanmar, Thailand, and Viet Nam; the GMS National Secretariats; co-financing and development partners; ADB and GMS Environment Operations Center (EOC) staff.