Permafrost is defined as ground that remains continuously at or below 0°C for at least two consecutive years; some 24% of the land surface in the Northern Hemisphere is classified as permafrost. In the northern high latitudes, strong warming has been observed over the recent decades, and climate models project strong future warming. A projected decline in the extent of permafrost will have a major impact on the Earth system, affecting global climate through the mobilization of carbon and nitrogen stored in permafrost. This special issue invites results of the large-scale European project PAGE21 with the aim to quantify the vulnerability of permafrost environments to a changing global climate, and to investigate the feedback mechanisms associated with increasing greenhouse gas emissions from permafrost zones. The focus is on (i) the combination of field mapping and measurements of permafrost landforms, ground ice content, processes, pools, and fluxes, with remote sensing data and global climate models at local, regional, and pan-Arctic scales, as well as (ii) advancing our understanding of permafrost processes at multiple scales, resulting in improvements in global numerical permafrost modeling.