One task of macroeconomics is to understand and predict fluctuations of economic activity over time. Economists seek to identify the moving forces of aggregate activity and to describe how these lead to changes in employment and output. Some of these forces, such as military spending, are accepted as fundamental by almost all economists. But there are also important moving forces that appear within a given model as "noise", that is, as departures of economic behaviour from the structural relations of the model. In this book a leading macroeconomist discusses several aspects of a noisy economy in order to refine our understanding of booms and recessions. This book contains three essays based on Arthur M. Okun Memorial Lectures that the author delivered at Yale University in 1988. It provides insights for better understanding business cycles and forming useful government policies.