This thesis discusses the role of agents expectations regarding the conduction of monetary and fiscal policies in determining policy outcomes, economic dynamics and the volatilities of macroeconomic variables. The first Chapter shows that accounting for agents’ expectations of a possible regime change has critical effects in the responses of macroeconomic variables to shocks, even if this switch does not materialize itself along the path observed after the shock. Recognizing the possibility of regime switches also have important consequences for the volatilities of endogenous variables, which are higher than those obtained in the linear model and very dependent on the policy parameters chosen by monetary and fiscal authorities in each regime. In the second Chapter, I discuss the role of expectations in determining the depth of a crisis when the economy hits the zero lower bound on nominal interest rates. I show that when analysing the impact of a fiscal stimulus during a zero interest rate episode, there is more than just short-run multipliers. To have larger positive effects on output and inflation, monetary and fiscal policies should last longer than the duration of the shock and be coordinated in their actions. The third Chapter presents a thoughtful evaluation of a fiscal stimulus in terms of the implied welfare losses making clear that it should account not only for the effects of policies on short-run output and inflation, but also for the present discounted value of output and inflation in future periods as well. It also analyses how to obtain the optimal level for the nominal interest rate once the economy gets out of the crisis state, if the monetary authority wants to use the expectations channel to undermine the depth of the crisis.