The article looks at L.S. Vygotsky's and P.Ya. Galperin's interpretations of one of the fundamental problems of psychology - relation between education and mental development. A historical genetic study conducted by the author shows that L.S. Vygotsky and P.Ya. Galperin had the same theoretical approach to the problem, but concentrated on different aspects of it. The main problem for Vygotsky was what to teach and he demonstrated that true teaching leads development. Galperin studied what and how to teach and he discovered correlation between the type of instruction and the child's mental development.