This paper examines the role of theory in public procurement research. Theoretical rigour is integral to management science, yet little is known on the extent and form of theory in public procurement. With the field starting to mature, addressing this issue is timely. From conducting a systematic literature review we find that 29 percent of articles are theoretically grounded, with the incidence of theory having increased in recent years. Economic, sociological, psychological, and management theories are all in evidence, but micro-economic theories predominate. Our findings also show that survey reporting and case studies account for almost half of all studies; procurement research is focused on organizational-level aspects more than regulatory-policy issues or public buyers; and studies to date have largely emanated from the North American and European regions. The contribution of this paper lies in clarifying the theoretical underpinnings of public procurement. Out of this we highlight the need for greater theoretical rigour, point to the under-use and even absence of theories that could have high validity and utility, and suggest a narrowing of research foci.