For almost two centuries economists have recognized that changes in the quantity of money have a delayed or lagged effect on wages, output, prices or gold flows. Recently there have been a number of attempts to measure some of the lags and to suggest the importance of the lags for monetary policy. I will make no attempt to summarize each of the many studies or to comment on the variety of procedures that have been used to generate particular estimates. Instead, I will discuss sons of the main conclusions and their relation to the problem of conducting monetary policy.