This essay studies the fundamental causes of the monetary policy regime switches within rational expectations models. I introduce a threshold-switching monetary policy process into the model that links the policy stance to the fundamental shocks by an autoregressive regime strength index. It creates an expectations feedback mechanism between private agents' policy forecasts and future policy regime outcomes. As demonstrated in a novel threshold-switching Fisherian model, well management of the

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... rivate sector's expectations of policy regime change can have the same effect as actually switching the regime. Contrastingly, failure to do so leads to unfavorable outcomes of policy intention. Then, I embed the new mechanism into a New Keynesian model. Along the way, I also develop an efficient non-simulation based threshold-switching Kalman filter, in conjunction with a solution method that accounts for the endogeneity of switching regimes, to estimate the nonlinear New Keynesian model. My key empirical findings are threefold. First, non-policy shocks have been instrumental in driving U.S. monetary policy regime changes during the post-World War II period. Most notably, markup shock explains 65.6% of variations in the policy regimes. Second, absent from markup shocks, eight of the eleven less aggressive regimes would not have happened during this history. Finally, I conclude that linking the private sector's dynamic expectations formation and the Fed's dilemma of the dual mandate in the presence of adverse supply shocks is a promising path towards providing micro-foundations for monetary policy regime shifts.