Activity theory (AT; Russian: Теория деятельности) is an umbrella term for a line of eclectic social-sciences theories and research with its roots in the Soviet psychological activity theory pioneered by Sergei Rubinstein in the 1930s. It was later advocated for and popularized by Alexei Leont'ev. Some of the traces of the theory in its inception can also be found in a few works of Lev Vygotsky. These scholars sought to understand human activities as systemic and socially situated phenomena and to go beyond paradigms of reflexology (the teaching of Vladimir Bekhterev and his followers) and classical conditioning (the teaching of Ivan Pavlov and his school), psychoanalysis and behaviorism. It became one of the major psychological approaches in the former USSR, being widely used in both theoretical and applied psychology, and in education, professional training, ergonomics, social psychology and work psychology.

Activity theory is more of a descriptive meta-theory or framework than a predictive theory. It considers an entire work/activity system (including teams, organizations, etc.) beyond just one actor or user. It accounts for environment, history of the person, culture, role of the artifact, motivations, and complexity of real-life activity. One of the strengths of AT is that it bridges the gap between the individual subject and the social reality—it studies both through the mediating activity. The unit of analysis in AT is the concept of object-oriented, collective and culturally mediated human activity, or activity system. This system includes the object (or objective), subject, mediating artifacts (signs and tools), rules, community and division of labor. The motive for the activity in AT is created through the tensions and contradictions within the elements of the system. According to ethnographer Bonnie Nardi, a leading theorist in AT, activity theory "focuses on practice, which obviates the need to distinguish 'applied' from 'pure' science—understanding everyday practice in the real world is the very objective of scientific practice. ... The object of activity theory is to understand the unity of consciousness and activity." Sometimes called "Cultural-Historical Activity Theory", this approach is particularly useful for studying a group that exists "largely in virtual form, its communications mediated largely through electronic and printed texts." Cultural-Historical Activity Theory has accordingly also been applied to genre theory within writing studies to consider how quasi-stabilized forms of communication regularize relations and work while forming communally shared knowledge and values in both educational and workplace settings.

AT is particularly useful as a lens in qualitative research methodologies (e.g., ethnography, case study). AT provides a method of understanding and analyzing a phenomenon, finding patterns and making inferences across interactions, describing phenomena and presenting phenomena through a built-in language and rhetoric. A particular activity is a goal-directed or purposeful interaction of a subject with an object through the use of tools. These tools are exteriorized forms of mental processes manifested in constructs, whether physical or psychological. As a result the notion of tools in AT is broad and can involve stationary, digital devices, library materials, or even physical meeting spaces. AT recognizes the internalization and externalization of cognitive processes involved in the use of tools, as well as the transformation or development that results from the interaction.

The origins of activity theory can be traced to several sources, which have subsequently given rise to various complementary and intertwined strands of development. This account will focus on three of the most important of these strands. The first is associated with the Moscow Institute of Psychology and in particular the "troika" of young Russian researchers, Vygotsky, Leont'ev and Luria. Vygotsky founded cultural-historical psychology, a field that became the basis for modern AT; Leont'ev, one of the principal founders of activity theory, both developed and reacted against Vygotsky's work. Leont'ev's formulation of general activity theory is currently a strong influence in post-Soviet developments in AT, which have largely been in social-scientific, organizational, and writing-studies rather than psychological research and organization.

The second major line of development within activity theory involves Russian scientists, such as P. K. Anokhin and Nikolai Bernstein, more directly concerned with the neurophysiological basis of activity; its foundation is associated with the Soviet philosopher of psychology Sergei Rubinstein. This work was subsequently developed by researchers such as Pushkin, Zinchenko & Gordeeva, Ponomarenko, Zarakovsky and others, and is currently most well-known through the work on systemic-structural activity theory being carried out by G. Z. Bedny and his associates, including a focus on the application of this theory as well as other related theories.

Finally, in the Western world, discussions and use of AT are primarily framed within the Scandinavian activity theory strand, developed by Yrjö Engeström.

After Vygotsky's early death, Leont'ev became the leader of the research group nowadays known as the Kharkov School of Psychology and extended Vygotsky's research framework in significantly new ways. Leont'ev first examined the psychology of animals, looking at the different degrees to which animals can be said to have mental processes. He concluded that Pavlov's reflexionism was not a sufficient explanation of animal behaviour and that animals have an active relation to reality, which he called "activity". In particular, the behaviour of higher primates such as chimpanzees could only be explained by the ape's formation of multi-phase plans using tools.^{[citation needed]}

Leont'ev then progressed to humans and pointed out that people engage in "actions" that do not in themselves satisfy a need, but contribute towards the eventual satisfaction of a need. Often, these actions only make sense in a social context of a shared work activity. This led him to a distinction between "activities", which satisfy a need, and the "actions" that constitute the activities. Leont'ev also argued that the activity in which a person is involved is reflected in their mental activity, that is (as he puts it) material reality is "presented" to consciousness, but only in its vital meaning or significance.^{[citation needed]}