Activity theory has long been an influential framework in the field of education. However, its theoretical concepts are not easily grasped by scholars, mainly due to difficulties in translation from the original Russian works, the complexity of these concepts and multiple versions embedded within the tradition. The two major approaches within activity theory were established by Leontiev and another version proposed later by Engeström, and they have often been confused and conflated together in the literature. This paper provides a much-needed theoretical comparison between these approaches in regard to the field of education. The criteria for comparison involve their theoretical foundations, central phenomena of interest, key theoretical concepts, units of analysis and explanatory mechanisms. Insights from this paper contribute to establishing a more refined understanding of activity theory and its variants, which in turn allows researchers to make more informed decisions when selecting and using these frameworks. Implications for practical research practices are discussed with examples from the area of teacher professional development.