Our understanding of the physical and biogeochem- ical processes at play in permafrost areas has been greatly inadequate and significant gaps have existed in our current knowledge, hindering any accurate as- sessment of the vulnerability of Arctic permafrost to climate change, or of the implications of future climate change for global greenhouse gas (GHG) emis- sions. The PAGE21 project was designed to broaden our knowledge base by assessing the vulnerability of Arctic permafrost to climate change and by reducing the uncertainties in permafrost-related feedbacks to the global climate. In particular, the PAGE21 project has sought to answer the following questions: • What are the key processes and parameters in- fluencing and controlling the vulnerability of the carbon and nitrogen pools in Arctic permafrost to future climate change? • How large an effect will Arctic climate change feedback (due to changes in the carbon and ni- trogen pools contained in permafrost) have on anthropogenic global warming? The concept of PAGE21 was to address these ques- tions through a close interaction between monitoring activities, process studies and modeling on the per- tinent temporal and spatial scales. Field sites were selected to cover a wide range of environmental condi- tions for the validation of large scale models, the de- velopment of permafrost monitoring capabilities, the study of permafrost processes, and for overlap with existing monitoring programs. Interaction between site-scale studies and large-scale modeling was de- signed to establish and maintain a direct link between these two areas for developing and evaluating, on all spatial scales, the land-surface modules of lead- ing European global climate models taking part in the Coupled Model Intercomparison Project Phase 5 (CMIP5), designed to inform the IPCC process. The PAGE21 results to date have highlighted that the amount of carbon released from permafrost over the 21st century may not be as large as previously claimed. The project has found an average loss of about 50 Gt C by 2100, which is at the lower end of the IPCC estimate of 50 to 250 Gt C. However, we have found that, in a much warmer world, this carbon loss would continue over hundreds of years leading to the severe degradation of permafrost ecosystems and a large cumulative loss of previously frozen carbon