What do we know about macroeconomics that Fisher and Wicksell did not?

This essay argues that the history of macroeconomics during the twentieth c entury can be divided into three epochs. Pre-1940: a period of exploration, during which all the right ingredients were developed. But also a period w here confusion reigned, because of the lack of an integrated framework. Fro m 1940 to 1980: a period during which an integrated framework was developed -from the IS-LM to dynamic general equilibrium models. But a construction w ith an Achilles' heel, too casual a treatment of imperfections, leading to a crisis in the late 1970s. Since 1980: a new period of exploration, focuse d on the role of imperfections in macroeconomics. Exploration often feels l ike confusion. But behind it is one of the most productive periods of resea rch in macroeconomics.