This chapter summarizes theory and research descended from Vygotsky and his followers that takes seriously the idea that practice is essential for testing and improving "cultural-historical activity theory" (CH/AT). It reviews some theoretical principles used in CH/AT-inspired intervention research. As applied to the domain of mathematics, the Elkonin-Davydov curriculum is designed to provide students with the clearest possible understanding of the concept of real number. Davydov's work was initially a key inspiration for the Finnish group of activity theorists who have expanded the use of the theory to the world of work. The work led to an intervention toolkit based on the principle of double stimulation. The chapter also discusses the 5th Dimension idiocultures, which routinely create an institutionalized version of a zone of proximal development for participants. Like the Change Laboratories, those who would use the 5th Dimension to challenge CH/AT theories turn to "real life" measures of effectiveness.