This article discusses the properties and occurrence of an active layer (AL) in the near-surface of the lithosphere in glacial and periglacial environments. This layer shows a seasonal variability in temperature, as a result of the climate. The AL, as classically understood, seasonally thaws and freezes, while in glacial environments it usually only reaches 0 °C. The definition of AL is currently not consistent with the definition of permafrost, even though both concepts usually appear linked. For these terms to be comparable, both should be defined based on temperature variability and not exclusively on phase change. Thus, the AL would be described not only as the upper section of perennially frozen ground presenting seasonal thaw-freeze cycles (# 1) but as a layer presenting a seasonal variation in temperature (# 2). Classical active layer can be thawed to a depth of approximately 2–8 cm, the thickest AL reaches over 20 m. In the particularly favorable conditions AL might be completely absent with the permafrost beginning at the ground surface. In glacial and sub-marine permafrost environments, the AL includes a layer of liquid water that seasonally accompanies the permafrost. Glaciers and ice sheets are usually devoid of the classical AL. In both cases, the AL is usually horizontal, but in specific terrains such as sea shore cliffs or karst environments, the AL may have a vertical course and may even be reversed. Both AL and permafrost are common in other frozen bodies in the solar system, differing mainly in their thermal character.