A major advance toward understanding the forces that lead to inflation and unemployment, this study takes a cost-benefit approach to monetary planning. Professor Phelps views a reduction of inflation by monetary means as a social investment and argues that such an investment must be subject to cost-benefit tests. By extending his analysis into questions of efficiency and resource allocation, he integrates the modern microeconomic theory of unemployment with the theory of optimal inflation. Another element of the analysis, the doctrine of optimum liquidity, is reassessed and found to require important revisions.