Revisiting the theory of institutional hybridity and diversity developed by Vincent and Elinor Ostrom to cope with the challenge of the “neither states nor markets” institutional domain, this article reconstructs the Ostromian system along the “value heterogeneity–co-production–polycentricity” axis. It articulates the elements of a theory of value heterogeneity and of the fuzzy boundaries between private and public. It rebuilds the model of co-production, clarifying the ambiguity surrounding a key technical public choice theoretical assumption, and it demonstrates (a) why it should not be confused with the Alchian-Demsetz team production model and (b) how co-production engenders a type of market failure that has been neglected so far. In light of this analysis, the article reconsiders polycentricity, the capstone of the Ostromian system, explaining why polycentricity may be seen as a solution both to this co-production market failure problem and to the problems of social choice in conditions of deep heterogeneity. It also discusses further normative corollaries.