The Circumpolar Active Layer Monitoring (CALM) program is one of several global-change programs affiliated with the International Permafrost Association (IPA). CALM was initiated in the early 1990s to track possible changes and trends in the seasonally frozen (“active”) layer in the permafrost regions. Widespread, large-magnitude increases in the thickness of the active layer induced by climatic warming could liberate carbon sequestered in near-surface permafrost, create irregular topography (“thermokarst terrain”) in areas of ice-rich permafrost, damage human infrastructure on the surface, and induce pronounced ecological changes. CALM is a hypothesis-driven program that monitors active-layer thickness and shallow ground temperature, coordinates field experiments, and provides data for use by investigators involved in a wide-range of cold-environment research and modeling activities. The CALM network is currently comprised of about 125 sites distributed throughout the Arctic, parts of Antarctica, and several mountain ranges of the midlatitudes. Efforts to expand the number and capabilities of sites in the Southern Hemisphere (CALM-S) are underway. Instrumentation and data-acquisition methods include monitoring the soil thermal and moisture regimes with automatic data loggers, mechanical probing of the seasonally thawed layer at specified spatial and temporal intervals, frost/thaw tubes, and a variety of instruments for measuring frost heave and thaw subsidence. Several groups of sites have been used to create maps of active-layer thickness, and estimates of the volume of thawed soil at regional scales. The CALM network has also provided a large amount of data pertaining to cryostratigraphy, cryoturbation, and soil carbon. Data obtained from the network have been used in validation procedures for hydrological, ecological, and climatic models, at a variety of geographic scales. Data are archived at the Frozen Ground Data Center (http://nsidc.org/fgdc/) in Boulder, Colorado. CALM is sponsored by the U.S. National Science Foundation's Office of Polar Programs. CALM is linked with many other global-change programs through the network of observatories known collectively as the Global Terrestrial Network for Permafrost (GTN-P), a network under the WMO Global Climate Observing Network (GCOS). With its sister programs Thermal State of Permafrost (TSP), Carbon Pools in Permafrost Regions (CAPP), and Arctic Coastal Dynamics (ACD), CALM forms a comprehensive effort on the part of the International Permafrost Association to monitor, understand, and predict the effects of environmental change in the world's permafrost regions. CALM is a major component of the IPA's coordinated program for the International Polar Year. Detailed information about the CALM program can be found at http://www.udel.edu/Geography/calm/.