Introduction to the Series The series Handbooks in Economics is to produce Handbooks for various branches of economics each of which is a definitive source reference and teaching supplement for use by professional researchers and advanced graduate students. Each handbook provides self contained surveys of the current state of a branch of economics in the form of chapters prepared by leading specialists on various aspects of this branch of economics. These surveys summarize not only received results but also newer developments from recent journal articles and discussion papers. Some original material is also included but the main goal is to provide comprehensive and accessible surveys.