Organic carbon stored in permafrost soils of the Arctic is one of the largest carbon reservoirs globally and is vulnerable to climate change. Despite its undisputed importance, the amount of arctic soil organic carbon (SOC) remains ambiguous and poorly co nstrained. A significant proportion of this SOC is stored in the subducted organic matter of Cryosols, suggesting that cryoturbation (i.e., the mixing of soil layers due to freezing and thawing) is one of the most important mechanisms of arctic carbon seq uestration. The major objectives of CryoCARB are therefore (i) to advance organic carbon estimates for cryoturbated soils including the carbon stored in the permafrost, (ii) to identify the major SOC stabilization mechanisms, focusing on organic matter qu ality, microbial community composition and on abiotic factors, and (iii) to assess the vulnerability of arctic carbon stocks in a future climate. CryoCARB is structured in work packages, dealing with carbon storage in cryoturbated soils (WP1), with qualit y and degradability of SOC (WP2), with microbial processes and community structure (WP3) and with the integrative modelling of SOC dynamics in cryoturbated soils (WP4). These WPs are linked and integrated by a set of joint sampling campaigns in Siberia (t hree transects), Greenland and Svalbard and by joint experiments that address the sensitivity of soil organic matter to changing environmental conditions in the laboratory (E1) and in the field (E2). The work will be supported by the development of a soun d theoretical and conceptual framework and by mathematical modelling. CryoCARB represents a multinational collaboration between 8 European countries and Russia, applying an interdisciplinary approach to address critically important questions linking cryot urbated arctic soils to the global carbon cycle. Such a comprehensive undertaking, from molecular microbiology to landscape level carbon inventories and modelling of circum-arctic carbon storage in future climates, is new and has not yet been implemented anywhere in the Arctic.