This book is an investigation of the methodological and epistemological foundations of macroeconomic theory, based on an examination of the theories of Keynes and Lucas. It is divided into two parts. In the first Professor Vercelli discusses the methodological issues which lie behind the conflict among different schools of thought in macroeconomics (equilibrium and disequilibrium, risk and uncertainty, rationality and causality). These issues are central to the current debate not only in many branches of economics, but also in other scientific disciplines. The traditional point of view of science based on equilibrium, stability and determinism has been increasingly challenged by a new point of view in which disequilibrium, instability and uncertainty play a crucial role. This, the author argues, is bound to put macroeconomics in a new, more promising position. In the second part of the book the author compares the two main alternative research programmes in macroeconomics: that outlined by Keynes in his 'General Theory', and that suggested by Lucas, the leader of the new classical economists.