The publication of volumes 3 and 4 of the Handbook of Public Economics affords us several opportunities: to address lacunae in the original two volumes of this series, to revisit topics on which there has been substantial new research, and to address topics that have grown in importance. Indeed, many of the papers individually encompass all three of these elements. For each chapter relates to one from an earlier volume, the new contribution is free standing, written with the knowledge that the reader retains the opportunity to review the earlier chapter to compare perspectives and consider material that the current author has chosen not to cover. Indeed, such comparisons illuminate the evolution of the field during the two decades that have elapsed since work first began on the chapters in volume 1. Taken together, the four volumes offer a comprehensive review of research in public economics over the past few decades, written by many of the field's leading researchers.