L.V. Zankov (1901-1977), a student and colleague of Vygotsky, developed a system of developmental education for elementary education through 20 years of extensive research. Since 2009, a number of Norwegian schools have been using textbooks in mathematics based on Zankov's system. In this article, we present and discuss the experiences of some Norwegian teachers -- both what they have learned about teaching mathematics by adopting the system and the challenges they have faced. We also outline some key theoretical points of Zankov's system and present illustrative example tasks from the textbooks used in Norway.