AbstractA brief summary is made of various field studies from Fennoscandia concerned with the origin of palsas, which are peat hummocks with a permafrost core rising above the mire surface. An insulating peat layer is important for preserving the frozen core of the palsa during the summer. Palsa formation needs low winter temperatures to form thick layers of frost, together with water-saturated peat and a thin snow cover. Factors limiting the size of palsas are discussed. Sharp, steep edges, which collect drifting snow, stop the areal growth of palsas. A model for cyclic palsa development is presented.