Mass killings under communist regimes occurred through a variety of means during the 20th century, including executions, famine, deaths through forced labour, deportation, starvation, and imprisonment. Some of these events have been classified as genocides or crimes against humanity. Other terms have been used to describe these events, including classicide, democide, red holocaust, and politicide. The mass killings have been studied by authors and academics and several of them have postulated the potential causes of these killings along with the factors which were associated with them. Some authors have tabulated a total death toll, consisting of all of the excess deaths which cumulatively occurred under the rule of communist states, but these death toll estimates have been criticized. Most frequently, the states and events which are studied and included in death toll estimates are the Holodomor and the Great Purge in the Soviet Union, the Great Chinese Famine and the Cultural Revolution in the People's Republic of China, and the Cambodian genocide in Democratic Kampuchea (now Cambodia) .
The concept of connecting disparate killings to the status of the communist states which committed them and the concept of trying to ascribe common causes and factors to them have both been supported and criticized by the academic community. Some academics view it as an indictment of communism as an ideology, while other academics view it as being overly simplistic and they also view it as being rooted in anti-communism. Also, some academics attribute the causes of the killings to either the political systems or the leaders of the communist states. There is also debate over whether or not all the famines which occurred during the rule of communist states can be considered mass killings. Mass killings which were committed by communist states have been compared to killings which were committed by other types of states. Monuments to the victims of communism exist in almost all the capitals of Eastern Europe and one in Washington, D.