The primary goal of the Circumpolar Active Layer Monitoring (CALM) program is to observe the response of the active layer and near-surface permafrost to climate change over long (multi-decadal) time scales. The CALM observational network, established in the 1990s, observes the long-term response of the active layer and near-surface permafrost to changes and variations in climate at more than 200 sites in both hemispheres. CALM currently has participants from 15 countries. Majority of sites measure active-layer thickness on grids ranging from 1 ha to 1 kmÂ², and observe soil temperatures. Most sites in the CALM network are located in Arctic and Subarctic lowlands. Southern Hemisphere component (CALM-South) is being organized and currently includes sites in Antarctic and South America. The broader impacts of this project are derived from the hypothesis that widespread, systematic changes in the thickness of the active layer could have profound effects on the flux of greenhouse gases, on the human infrastructure in cold regions, and on landscape processes. It is therefore critical that observational and analytical procedures continue over decadal periods to assess trends and detect cumulative, long-term changes.