The field of public economics has been changing rapidly in recent years, and the 18 chapters contained in volumes three and four of this handbook, survey many of the new developments. As a field, public economics is defined by it's objectives rather than it's techniques and much of what is new is the application of modern methods of economic theory and econometrics to problems that have been addressed by economists for over 200 years. More generally the discussion of public finance issues also involves elements of political science, finance and philosophy. These connections are evidence in several of the chapters that follow. Public economics is the positive and normative study of government's effect on the economy. This volume attempts to explain why the government behaves as it does, how its behaviour influences the behaviour of private firms and households, and what the welfare effects of such changes in behaviour are.