Activity theory
Activity theory (AT; ) 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 Lev Vygotsky , Alexei Leont'ev and Sergei Rubinstein. 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.
Cultural-historical activity theory
The rapid rise of new forms of activities characterised by web-based social and participatory practices, 
, phenomena such as distributed workforce and the dominance of knowledge work, prompts a rethink of the third generation model, bringing with it the need, as suggested by Engestrōm, for a new, "fourth generation" activity system model. which activity theorists indeed have been working on in recent years. In fourth generation CHAT, the object(ive) will typically comprise multiple perspectives and contexts and be inherently transient; collaborations between actors, too, are likely to be temporary, with multiple boundary crossings between interrelated activities. Fourth-generation activity theorists have specifically developed activity theory to better accommodate Castells's (and others') insights into how work organization has shifted in the network society: they hence will focus less on the workings of individual activity systems (often represented by triangles) and more on the interactions across activity systems functioning in networks. The 2017 ISCAR congress (August, Quebec City) has the following theme: ""Taking a 360° view of the landscape of cultural-historical activity research:The state of our scholarship in practice".