Enforcement of any rule or regulation is where 'the rubber hits the road'. Many economists and policy analysts have been guilty of proposing and promoting legal and regulatory instruments having given scant or no regard to the problems that might surround their implementation. Having said this, a significant and rapidly growing economics literature (both theoretical and empirical) has sought to think through some of the practical issues of implementation. Many of its conclusions for policy are not always obvious, and sometimes downright surprising.