The Russian psychologist Lev Vygotsky (1896–1934) was one of the most influential psychologists of the 20th century. As a psychologist following in his footsteps, Piotr Gal'perin (1902–1988) could justifiably be called a Vygotskian, but he departed from Vygotsky's sociocultural theory and developed its educational implications. The formative role of education is significant in Vygotsky's notion of a zone of proximal development (ZPD). Gal'perin extended Vygotsky's “zone” to also include a teaching–learning model of the formation of mental actions. This model integrates the notions of mediation, activity and internalization. In doing so, Gal'perin outlines steps in the teaching–learning process, formulates a set of conditions for the development of mental actions and shows the teacher's role. This article summarizes Gal'perin's learning–psychological theory and discusses its use in today's classrooms.