SUMMARYConsiderable controversy surrounds the COASE theorem despite the fact that it is generally accepted as valid both in the theory of externalities and the economic analysis of law. A major source of the controversy is the fact that there are a number of versions of the COASE theorem and definitions of transactions costs. It is argued that a proper understanding both of the competitive market model and transaction costs reveals that the COASE theorem is invalid and not part of standard market theory. It is further argued that transaction costs and information costs arising from market uncertainty provide a key to resolving many of the controversies that the theorem has given rise to and following from this that the primary economic function of law in bargaining situations is to increase transaction costs in order to reduce market uncertainty and strategic behaviour.