This paper presents a general equilibrium analysis of economic efficiency under externalities, transaction costs and non-convexity. It applies to market exchange as well as contractual arrangements. We show that the Coase theorem continues to hold under general conditions: the efficient management of externalities remains consistent with aggregate profit maximization under transaction costs and non-convexity. We examine the role of transaction costs and explore how the minimization of transaction costs is an integral part of efficient allocations. We also show how our analysis applies under non-convex technology, provided that we allow for non-linear pricing in markets.