Agile development means self-management, collaboration, and working towards shared goals. Agile practices support much of this, but we can still learn more, both to better understand current practices and to develop new ones. This session is an introduction to cultural-historical activity theory, a psychological framework for understand collaborative behaviour. The framework has shown the role of tools in cognition and collaboration, and understanding structural tension between different activity systems: it can also be used to understand and improve agile software development.