How ecosystem services are produced remains a critical research gap that must be addressed if services are to persist under on-going and future environmental change. Interest in this area is coalescing under the term ‘co-production’, which recognises that services are generated through interactions between social and ecological processes. Here we conduct a critical review of academic research into the co-production of ecosystem services, aiming to understand the foundations of this emergent field. Despite its recent origins, we identify four different framings as to how ecosystem services are co-produced and discuss their different epistemological bases and applications. The four framings are: input focused, which identifies and measures the inputs underpinning co-production; actor focused, understanding who is involved in co-production; context focused that situates co-production in social relations and place; and a more disparate disciplines focused approach, which highlights alternative conceptualisations of co-production based on diverse disciplinary and conceptual perspectives. There is overlap and dialogue between the four approaches, and we identify examples of how and where to operationalise these framings together to achieve a more holistic understanding of co-production processes. Nevertheless, behind these different framings are differences over what is or is not considered co-produced, and thus what is considered a valid field of inquiry within co-production research. This indicates ontological differences on the social construction of ecosystem services and the role of people therein. We argue that diversity in co-production research is important for representing the complexity of human-environment interactions, but that a more explicit acknowledgement of the ontological assumptions underpinning co-production is crucial if this area of research is to be analytically useful for the management of current and future ecosystem services.