EXECUTIVE SUMMARY — Given the importance of nominal bonds in investment portfolios, and in the design and execution of fiscal and monetary policy, financial economists and macroeconomists need to understand the determinants of nominal bond risks. This is particularly challenging because the risk characteristics of nominal bonds are not stable over time. In this paper the authors ask how monetary policy has contributed to these changes in bond risks. They propose a model that integrates the building blocks of a New Keynesian model into an asset pricing framework in which risk and consequently risk premia can vary in response to macroeconomic conditions. The model is calibrated to US data between 1960 and 2011, a period in which macroeconomic conditions, monetary policy, and bond risks have experienced significant changes. Findings show that two elements of monetary policy have been especially important drivers of bond risks during the last half century. First, a strong reaction of monetary policy to inflation shocks increases both the beta of nominal bonds and the volatility of nominal bond returns. Positive inflation shocks depress bond prices, while the increase in the Fed funds rate depresses output and stock prices. Second, an accommodating monetary policy that smooths nominal interest rates over time implies that positive shocks to long-term target inflation cause real interest rates to fall, driving up output and equity prices, and nominal long-term interest rates to increase, decreasing bond prices. The paper shows empirical evidence that the Fed monetary policy followed an anti-inflationary stance after 1979, but it has moved to a more accommodating, nominal interest rate smoothing policy since the mid 1990's. Consistent with the predictions of the model, the first period corresponds to a period of average positive Treasury-bond beta and stock-bond