The goal of this paper is identify the theoretical foundations – the core theories – of the IS field. Currently there is a lack of consensus about what the core IS theories are, or even if we have any at all. If we do, they certainly don’t appear in IS curricula or textbooks as they do in more mature disciplines. So far, most of the debate on this issue has been conducted at a subjective and prescriptive (normative) level. We attempt to broaden the debate by taking a descriptive (positive) approach, using relatively objective data. We do this by consulting the “geological record”: the pattern of citations in the leading IS journals. We use a combination of quantitative and qualitative techniques to identify the most influential theories in the IS field. The results of our analysis are surprisingly positive, especially in the light of warnings about IS being overly dependent on reference disciplines (a discipline with no theory to call its own) and being obsessed with research methodology (emphasising how to research at the expense of what to research). This suggests that the negative views often expressed about the progress of IS may be unjustified and that its development has followed the normal evolutionary pattern of any research field. Being aware of our theoretical foundations will help clarify our disciplinary identity and guide teaching and scholarship.