Abstract The long, cold winters and short, cool summers in the polar regions result in the formation of a layer of frozen ground that does not completely thaw during the year. This perennially frozen ground, known as permafrost, affects many human activities in the Arctic, as well as in the Subarctic and at high altitudes, and causes problems that are not experienced elsewhere. Permafrost is a naturally occurring material that has a temperature below 0°C continuously for two or more years (Muller, 1943, p. 3). This layer of frozen ground is designated exclusively on the basis of temperature. Part or all of its moisture may be unfrozen, depending upon the chemical composition of the water or depression of the freezing point by capillary forces. For example, permafrost with saline soil moisture, such as that found under the ocean immediately off the arctic shores, might be colder than 0°C for several years but would contain no ice and thus would not be firmly cemented. Most permafrost is consolidated by ice; permafrost with no water, and thus no ice, is termed dry permafrost. The upper surface of permafrost is called the permafrost table. In permafrost areas, the surficial layer of ground that freezes in the winter (seasonally frozen ground) and thaws in summer is called the active layer. The thickness of the active layer under most circumstances depends mainly on the moisture content; it varies from 10 to 20 cm in thickness in wet organic sediments to 2 to 3 m in well-drained gravels. Permafrost is a widespread phenomenon in the northern part of the Northern Hemisphere, underlying an estimated 20 percent of the land surface of the world (Fig. 1).