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 a potential link between the killings and the perpetrators' status as communist states. 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 criticised. 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). Estimates of individuals killed range from a low of 10–20 million to as high as 148 million.

The concept of connecting these killings to the ideology of the states that committed them has been both supported and criticized by the academic community. Some academics view this connection as causal in nature, and thus as an indictment of communism as an ideology, while other academics view such analyses as being overly simplistic and rooted in anti-communism. There is academic debate over whether the killings should be attributed to the political system, or primarily to the individual leaders of the communist states; similarly, there is debate over whether all the famines which occurred during the rule of communist states can be considered mass killings. Mass killings committed by communist states have been compared to killings which were committed by other types of states. Monuments to individuals and groups considered to be victims of communism exist in almost all the capitals of Eastern Europe, as well as many other cities in the world.

Several different terms are used to describe the intentional killing of large numbers of noncombatants. According to historian Anton Weiss-Wendt, the field of comparative genocide studies has very "little consensus on defining principles such as definition of genocide, typology, application of a comparative method, and timeframe." According to professor of economics Attiat Ott, mass killing has emerged as a "more straightforward" term.

The following terminology has been used by individual authors to describe mass killings of unarmed civilians by communist governments, individually or as a whole:

According to historian Klas-Göran Karlsson, discussions of the number of victims of communist regimes have been "extremely extensive and ideologically biased." Any attempt to estimate a total number of killings under communist regimes depends greatly on definitions, resulting in a vast range of estimates ranging from a low of 10–20 million to as high as 148 million. Political scientist Rudolph Rummel and historian Mark Bradley have written that, while the exact numbers have been in dispute, the order of magnitude is not. Professor Barbara Harff says that Rummel and other genocide scholars are focused primarily on establishing patterns and testing various theoretical explanations of genocides and mass killings. They work with large data sets that describe mass mortality events globally and have to rely on selective data provided by country experts; researchers cannot expect absolute precision, and it is not required as a result of their work.

The usefulness of using communist regimes as a category under which to count killings has been disputed. Historian Alexander Dallin argued that the idea to group together different countries such as Afghanistan and Hungary has no adequate explanation. During the Cold War era, some authors (Todd Culberston), dissidents (Aleksandr Solzhenitsyn), and anti-communists in general attempted to make both country-specific and global estimates. Scholars of communism have mainly focused on individual countries, while genocide scholars have attempted to provide a more global perspective, maintaining that their goal is not reliability but establishing patterns. Scholars of communism have generally debated on estimates for the Soviet Union instead of for all communist regimes, an practice which was popularized by the introduction to The Black Book of Communism but which was controversial. Among scholars of communism, Soviet specialists Michael Ellman and J. Arch Getty have criticized estimates for relying on émigré sources, hearsay, and rumor as evidence, and cautioned that historians should instead utilize archive material. Such scholars distinguish between historians who base their research on archive materials and those whose estimates are based on witnesses evidence and other data that they consider unreliable. Soviet specialist Stephen G. Wheatcroft says that historians relied on Solzhenitsyn to support their higher estimates but research in the state archives supported the lower estimates, and that the popular press has continued to include serious errors that should not be cited, or relied on, in academia. Rummel was also another widely used and cited source.

Notable estimate attempts include the following:

Criticism of the estimates is mostly focused on three aspects, namely that the estimates are based on sparse and incomplete data, making significant errors inevitable, that the figures are skewed to higher possible values, and that victims of civil wars, Holodomor and other famines, and wars involving communist governments should not be counted. Criticism also includes arguments that these estimates ignore lives saved by communist modernization and that they engage in comparisons and equations with Nazism, which are described by scholars as Holocaust obfuscation, Holocaust trivialization, and anti-communist oversimplifications. In addition, the communist grouping as applied by Courtois and Malia in The Black Book of Communism has been claimed to have no adequate explanation by historian Alexander Dallin, while Malia has been able to link disparate regimes, from radical Soviet industrialists to the anti-urbanists of the Khmer Rouge, under the guise of a "generic communism" category "defined everywhere down to the common denominator of party movements founded by intellectuals."