This chapter is about a theoretical and methodological approach to studying and working with learning and development in its broadest sense, commonly known as Activity Theory. The roots of this theory stem from Lev Vygotsky’s seminal work on the role of mediation, which was later developed by his students, amongst others Aleksei Leont’ev, and lately expanded by Yrjö Engeström to include networks ... [Show full abstract] of activities. The concept of expansive learning, learning something that is not yet there, is central in Activity Theory. Activity Theory can be applied to understand the principles of inquiry-based science teaching by developing various mediating artifacts. The example provided in this chapter is a model known as the 5E model, a mediating artifact in inquiry-based science teaching. This chapter outlines the contributions from three schools of Activity Theory, and shows the relevance of Activity Theory in working with development in general and inquiry-based science teaching in particular.