In the field of IS, researchers use and adapt existing theories to make sense of their data. They also build new theory from their research findings. The way theory is used, adapted or created usually assumes a certain unit of analysis, which could be the artefact, the system, the organisation, the user, the developer, the team or something else. In this paper we propose that ‘activity’ should also be considered as a suitable unit of analysis for theory in IS since the purpose of any information systems is to facilitate activities of use. To support this proposition, we describe tenets of Activity Theory and how they can be used to underpin IS research. We illustrate these with the interpretation, through Activity Theory, of a study of health information systems development aimed at identifying and meeting the needs of various users’ activities. We make the claim for activity as an appropriate unit of analysis in the use of existing theory in IS research and when building new theory for IS.