Arctic permafrost soils contain the largest terrestrial pool of organic carbon (C). Even partial mobilization of permafrost C may propel our current atmospheric greenhouse gas (GHG) concentrations into heights unknown in recent Earth history. Rapid thaw of ice-rich permafrost causes ground subsidence, thermokarst lake growth, talik formation, or thaw slumping. While the major effects of rapid permafrost thaw on heterotrophic respiration, methane release, and the potential for climate feedbacks have been recognized, they are not yet considered by Earth System Models (ESM). This creates large uncertainty in projections of C release from permafrost thaw and climate feedback estimates. Key challenges include (1) assessing permafrost thaw rates under diverse and changing environmental conditions on a panarctic scale; (2) analysing impacts of varied thaw processes on the heterogeneous permafrost C stock; and (3) integrating and scaling such dynamics using process models, and implementing these in ESM to project permafrost C trajecto- ries and climate feedbacks. PeTCaT, a new international research endeavour funded by Schmidt Sciences within their Virtual Institute of Carbon Cycle (VICC) program, will tackle these challenges with a team combining expertise on permafrost, soil C and GHG biogeochemistry, remote sensing, process modeling, and global climate modeling. In the 5-yr project we will address Rapid Thaw Processes, i.e., where, how fast, and how deep rapid thaw takes place and how at- mospheric changes will influence these dynamics in the future, as well as what the distribution of ground ice is; the Reactivity of Rapidly Thawing Organic C, i.e., where, how much, and what type of organic matter is thawed, how fast it decomposes under which conditions, and what the consequences for GHG fluxes are; and the Emission Contribution and Global Consequences, i.e., whether the strength of the permafrost carbon-climate feedback is currently underestimated, causing an overestimation of the remaining C budget.