A collection of articles on modern business cycle theory. Fundamental to business cycle theory is estimating the role played by different impulses or shocks for aggregate fluctuations, and identifying the mechanisms by which these impulses propogate over time to create the cycles we observe. The book is divided in to three parts. Part One deals with issues of measurement and methodology and describes the empirical business cycle regularities. Parts Two and Three centre around the study of shocks or impulses that primarily can be regarded as nominal (monetary and credit) and real (technology and fiscal policy).