This 21st edition of the NBER Macroeconomics Annual treats many questions at the cutting edge of macroeconomics that are central to current policy debates. The first four papers and discussions focus on such current macroeconomic issues as how structural-vector-autoregressions help identify sources of business cycle fluctuations and the evolution of U.S. macroeconomic policies. The last two papers analyze theoretical developments in optimal taxation policy and equilibrium yield curves.Daron Acemoglu is Charles P. Kindleberger Professor of Applied Economics at MIT. Kenneth Rogoff is Thomas D. Cabot Professor of Public Policy and Professor of Economics at Harvard University. Michael Woodford is John Bates Clark Professor of Political Economy at Columbia University. All three are Research Associates of the National Bureau of Economic Research.