This paper is concerned with decentralized models of market equilibrium in which the primary interest lies in the nature of the institutions that govern trade. There are two separate branches in this literature. In the first, the institutions that govern trade are taken to be given exogenously. The primary motivation for models in this class is essentially the study of comparative institutions. In particular the papers are interested in whether there are specific institutions that will generate Walrasian prices and allocations approximately when transactions costs are small. In the second branch of the literature, the institutions that govern trade themselves are made endogenous. These models characterize the nature of these endogenously generated institutions and examine their efficiency in environments where transactions costs are small.