Karl Marx's theory of alienation describes the social alienation (German: Entfremdung, lit. 'estrangement') of people from aspects of their human nature (Gattungswesen, 'species-essence') as a consequence of living in a society of stratified social classes. The alienation from the self is a consequence of being a mechanistic part of a social class, the condition of which estranges a person from their humanity.

The theoretical basis of alienation within the capitalist mode of production is that the worker invariably loses the ability to determine life and destiny when deprived of the right to think (conceive) of themselves as the director of their own actions; to determine the character of said actions; to define relationships with other people; and to own those items of value from goods and services, produced by their own labour. Although the worker is an autonomous, self-realized human being, as an economic entity this worker is directed to goals and diverted to activities that are dictated by the bourgeoisie—who own the means of production—in order to extract from the worker the maximum amount of surplus value in the course of business competition among industrialists.

In the Economic and Philosophic Manuscripts of 1844 (1932), Karl Marx expressed the Entfremdung theory—of estrangement from the self. Philosophically, the theory of Entfremdung relies upon The Essence of Christianity (1841) by Ludwig Feuerbach, which states that the idea of a supernatural god has alienated the natural characteristics of the human being. Moreover, Max Stirner extended Feuerbach's analysis in The Ego and its Own (1845) that even the idea of 'humanity' is an alienating concept for individuals to intellectually consider in its full philosophic implication. Marx and Friedrich Engels responded to these philosophical propositions in The German Ideology (1845).