Activity theory analyzes human behavior in social contexts. It originated from Vygotsky's notion of mediated action and the relationship between subject, object, and mediating artifact. Leont'ev introduced the distinction between individual action and collective activity, considering how division of labor impacts activity. Engeström expanded the model to include community, rules, division of labor, and how they interact within an activity system. Activity theory principles include the hierarchical structure of activity, its object-oriented nature, internalization/externalization of tools, mediation of human activity by tools, and development over time through contradictions. It has been applied to understand technology use in education and analyze changes within and between activity systems.