Modern law and economics received much of its impetus from Ronald Coase's analysis in ‘The Problem of Social Cost,’ and a goodly amount of that comes from the Coase theorem, which states that, absent transaction costs, externalities will be efficiently resolved through bargaining. The fact that the analysis that came to be codified in the Coase theorem was (intentionally) an exercise in pure fiction on Coase's part did not deter the erection of a substantial edifice of positive and normative analysis on this foundation, nor, for that matter, has subsequent elaboration of Coase's intent done anything to abate the interest in the theorem and its implications. © 1999, Cambridge University Press. All rights reserved.