Abstract
Activity theory has an extensive history in the Soviet Union dating back to the works of Vygotsky and his followers. Activity (or "deyatel'nost" in Russian) refers to a coherent system of internal mental processes and external behaviors and motivations that are combined and organized by the mechanisms of self-regulation to achieve a conscious goal. Activity theory is emerging as a new paradigm for psychology and an interdisciplinary approach to human sciences in Europe. Furthermore, scientists are writing about the internationalization of this approach. For example, activity theory exerted a great influence on the development of ACTION THEORY in Germany. In the United States, activity theory is associated with the sociocultural approach initiated by Vygotsky. However much data in this area remains unknown to the English-speaking world. This article introduces and discusses an important principle of Activity theory, that is, "the principle of unity of cognition and behavior." Because this principle is important to the study of work behavior, we believe that its introduction to the English-speaking scientific community will generate new ideas in the area of human performance.