This paper aims at contributing to our understanding of business-IT alignment and at offering a novel and theoretically well-grounded view on the phenomenon. Drawing on Activity Theory (AT), we argue that business and IT reflect two distinct, yet related activity systems. We propose that applying AT ena-bles systematic analysis of these systems, their constituents and complex interrelations. Building on AT’s notion of tensions, we further assume that AT enables us to identify and manage misalignment within and between the systems as well as drivers for their mutual development. Subsequently, AT provides a coherent framework for describing and analysing complex interrelations between both domains. To illustrate the applicability of AT and highlight its potentials to contribute to our understanding of business-IT alignment, we conducted a single, in-depth case study. As to that, we analyse six years of co-evolution of business and IT within a complex organizational setting. Our empirical examinations show that continually approaching emerging tensions within and between the two activity systems and implementing changes enables co-evolutionary processes of both systems. Though further research has to approve the practical applicability of the framework, we provide early evidence that activity system analysis can enable practitioners to purposefully detect and approach misalignments.