This thesis consists of three chapters on macroeconomic policy and business cy-cles. In chapter 1, I estimate a time-varying structural VAR to study the effects of government spending shocks on a number of U.S. macroeconomic variables. In contrast to the predictions of the standard New Keynesian models, I find no significant changes in the size of the government spending multiplier when the federal funds rate hits the Zero Lower bound (ZLB). I propose a theoretical model where the central bank, through either conventional or unconventional policies, directly controls the market interest rate, and where the policy rule parameters are subject to regime switches to capture potential changes due to the ZLB con-straint. The model estimates sugge...