After Vygotsky's early death, Leontiev became the leader of the activity theory research group and extended the framework in significantly new ways. Many of the specifics of activity theory set out below derive, at least in their original form, from Leontiev's work. Leontiev 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.

Leontiev 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 lead him to a distinction between activities, which satisfy a need, and the actions that constitute the activities.

Leontiev 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.