Primary production in lichens, as in all plants, is the result of gains and losses. Losses of carbon may be high under extreme environmental conditions. In the harsh climate of polar regions lichens use much carbon for maintenance and in their stress response. Thallus biomass is preserved through periods of darkness, extremely low temperatures and snow cover. On the other hand, lichens are photosynthetically active at temperatures below 0°C. They can make use of melted water from snow and ice and take up water vapour from snow. Physiological adaptations and environmental conditions under which growth is possible are discussed in terms of our recent results. A concept model describes the main parameters that influence carbon acquisition, growth and water relations of lichens in polar regions.