The most famous institution of Stalinist repression is undoubtedly the Gulag a word formed from the acronym for Main Administration of Camps in Russian. Alexander Solzhenitsyn's Gulag Archipelago published in 1973 popularized the term in the US and Europe. In popular use the term Gulag refers to the system of corrective labor camps (ITLs) and corrective labor colonies (ITKs) under Stalin. Due to substandard living conditions including poor housing, insufficient nutrition, physically demanding and dangerous labor, lack of proper sanitation and routine brutality by guards and other inmates this system killed a very large number of people. The figures for deaths from these causes in the Gulag are incomplete. They do not include deaths during transit or people who died shortly after being released from the camps for instance. The practice of releasing prisoners on the verge of deaths from the camps was fairly common.
The total number of recorded fatalities in ITLs and ITKs from 1930 to 1956 is 1,606,748 according to SANO (GULag medical department). A full 1,053,829 of these deaths took place in ITLs from 1934 to 1953. Since these figures are incomplete it is not far fetched to assume that the actual number of deaths due to incarceration in the USSR during this time is significantly higher. How much higher is a matter of speculation. Anne Applebaum suggests that the figure should be nearly doubled to 3 million to take into account unrecorded deaths.
Further discussions of the numbers involved in the Gulag can be found in the following works.
Anne Applebaum, Gulag: A History (NY: Doubleday, 2003).
Oleg V. Khlevniuk, A History of the Gulag: From Collectivization to the Great Terror (Yale University Press, 2004).
J. Otto Pohl, The Stalinist Penal System (Jefferson, NC: McFarland, 1997).