THE QUESTION of how monetary policy affects the real economy is a perennial one in macroeconomics. Over the past several decades, however, the focus of the debate has changed. Today it is taken for granted that monetary policy affects aggregate demand; what is debated is why prices do not adjust fully to compensate for shifts in demand. Thirty years ago, in contrast, sluggish price adjustment was taken for granted; what was debated was the magnitude of the effect of monetary policy on aggregate demand and the channels through which that effect occurred. This paper returns to the subject of that older literature. A fresh look at the way monetary policy affects aggregate demand is particularly timely in light of recent developments in theoretical analyses of credit markets. Work over the past 15 years has suggested that imperfections are a central feature of capital markets, and that these imperfections can cause credit allocation to be made largely on the basis of quantity rationing rather than price adjustment and can create a special role for lending by financial intermediaries. This work has also shown that credit market imperfections can have important consequences for macroeconomic fluctuations in general and for the way monetary policy is transmitted to aggregate demand in particular.