My dissertation studies and quantifies the implications of various expectations formation processes for what concerns macroeconomic fluctuations and monetary policy transmission. The first chapter (joint work with Marco Airaudo) studies the existence of Stochastic Consistent Expectations Equilibria (SCEE) in linear Markov regime switching models. A SCEE exists when the model-implied mean and first order autocorrelation coincide with those predicted by the agents via misspecified forecasting rules. For a simple regime-switching monetary policy model, the parametric space where at least one SCEE exists is rather wide, and may extend well beyond the rational expectations equilibrium determinacy frontier. Misspecified expectations combined with regime-switching yield a strong endogenous amplification mechanism that help generate the near unit root dynamics for inflation observed in the U.S. before the Great Moderation. The second chapter considers a New Keynesian model in which agents form expectations based on a combination of misspecified forecasts and myopia. The proposed expectations formation process is tested against Rational Expectations (RE), as well other assumptions about expectations, with inflation forecasting data from the U.S. Survey of Professional Forecasters. The paper then derives the general equilibrium solution consistent with the proposed expectations formation process and estimates the model with likelihood-based Bayesian methods. The paper yields three novel results: (i) Datastrongly prefer the combination of autoregressive misspecified forecasting rules and myopia over other alternatives, including RE; (ii) The best fitting expectations formation process for both households and firms is characterized by high degrees of myopia and simple AR(1) forecasting rules; (iii) Despite the absence of real rigidities typically found necessary for New Keynesian models with RE, the estimated model with autoregressive forecasts and myopia generates substantial internal persistence and amplification to exogenous shocks. The third chapter proves that in Full-Information RE models with exogenous Markov regime shifts, ex-post regime-dependent forecasting errors can be described by available information at the time of forecast and ex-ante forecasting revisions, separately. In economic environments with structural changes, the FIRE hypothesis gives rise to waves of over-and under-response of forecasters to current events as well as new aggregate information at the time of forecast. Using inflation and output growth forecasting data from the Survey of Professional Forecasters, the paper presents new evidence of such waves, consistent with implications of Full-Information RE in models with regime shifts. Finally, the framework and insights are generalized to any dynamic stochastic general equilibrium model with exogenous Markov shifts, whose RE solution can be written as a Markov Switching VAR process.