This book presents current knowledge on aggregate economic fluctuations -- business cycles. The book begins with an empirical definition of the 'business cycle'. After outlining the historical development and key features of the current theories of the business cycle, these theories are evaluated on the basis of their ability to explain the facts. Included in this evaluation is a review of whether and how the competing theories could be distinguished empirically. The book concludes by highlighting important theoretical and empirical issues for future business-cycle research. Contributors include: Victor Zarnowitz and Arnold Zellner/University of Chicago; Ben Bernanke and Alan Blinder/Princeton; Michael R. Darby/Ucla; Ray Fair/Yale; Benjamin Friedman and James Stock/Harvard; Herschel Grossman/Brown University; and David Laidler and Michael Parkin/University of Western Ontario.