The theories and debates considered to be "classical Marxism" are those that were already being discussed during the lifetimes of Karl Marx and Friedrich Engels, in the period of early socialism and within the socialist labour movement, particularly within its organisations and political parties. The term "classical Marxism" is used here to specify a certain temporal and logical field; it does not reflect a value judgement.
Three generations can be distinguished within "classical Marxism." The first comprises the theories of early socialism and the debates conducted during the lifetimes of Karl Marx and Friedrich Engels. The Marxism of the Second International (1889–1914/1918) constitutes a second generation, following which "Marxism-Leninism" (ML) developed into a state doctrine – first in the Soviet Union, following Lenin's death, then in the Third International and, later still, in the real socialist states.
Within this third generation, however, a rupture within Marxism becomes evident. For while Marxism-Leninism situated itself within the tradition of the "classics" and claimed to engage in their "creative application," the genuine advancement of Marxist theories occurred elsewhere: first in "Austro-Marxism" and later in left socialism and left communism, which are treated as variants of "Classical Marxism" here. Thereafter, a Marx-based critique of society developed and renewed itself mainly within those currents that are summarised in the second major category, namely "Western and Heterodox Marxism." On the other hand, some later 20th century currents, such as Trotskyism and Maoism, are attributed to "Classical Marxism".