Palsa mires are northern mire complexes with permanently frozen peat hummocks. These are degrading throughout their distribution range, probably because of regional climatic warming. This review of the current understanding of the geographical, climatic and biological characteristics of palsa mires focuses on Europe. Recent studies have reported a drastic decrease in the extent of palsa mires in Fennoscandia; in Finland, the distribution of palsas was formerly about three times that at present. With continued or accelerated warming, as predicted for high latitudes, further extensive degradation or the wholesale disappearance of palsa mires seems inevitable. Palsa mires are known to be biologically heterogeneous environments with a rich diversity of bird species, and they are listed as a priority habitat type by the European Union. However, their role as habitats for other organisms is still poorly understood. There is urgent need for research and monitoring to assess the ecological and biological consequences of the decline of palsa mires in Europe.