The Global Terrestrial Network for Permafrost (GTN-P) is the primary international programme concerned with sustained long-term monitoring of permafrost. GTN-P was developed in the 1990s by the International Permafrost Association (IPA) under the Global Terrestrial Observing System (GTOS) as part of the Global Climate Observing System (GCOS). The two major components of GTN-P (Essential Climate Variables) are: (a) long-term monitoring of the thermal state of permafrost in an extensive borehole network, the Thermal State of Permafrost - TSP; and (b) monitoring of the Active-layer thickness - ALT. Long-term monitoring of permafrost generates essential baseline information for the assessment of climate change impacts in polar and high mountain regions.