Strong population cycles characterise the dynamics of arvicoline rodents (voles and lemmings} in northern Fennoscandia. The peaks and migrations of Norway lemmings are the epitome of these fluctuations. Lemming peaks coincide with those of voles but there is not a lemming peak during every vole peak. Favorable winter conditions, soft powder snow and absence of warm spells, contribute to lemming peaks. There is a gradient in lemming dynamics from high mountains to lower ones and taiga. Peaks occur more often in higher mountains. These are characterised by high proportion of optimal wintering habitats, and winter breeding is essential for the population increase of lemmings. Therefore, lemmings are sensitive to the climate change. During great peaks lemmings can migrate deep into the boreal taiga zone, undergo few local cycles there in the rhythm of the local vole cycle, and then disappear. These large-scale migratory periods occur seldom. In Finnish Lapland, these major events, lemmings migrating 200 km into the taiga, occur twice or three times in a century. These kinds of large-scale movements need 2-3 years to develop and expand. The last great migration in Finnish Lapland occurred in 1969-70, even though some moderate migrations have taken place more recently. In the taiga lemmings encounter totally a new predator and parasite-pathogen world, which they may not have adapted to. The outbreak character and magnitude of lemming dynamics depends much on winter conditions, and how this affects the timing of lemming increase in relation of lowland vole dynamics, i.e. varying predation pressure.