Economic agents adapt to expected and unexpected shocks in their decision-making. This thesis develops three theoretical chapters about the business cycle, and studies three different sets of economic activities through numerical analysis. Chapter 1 studies the consumption and saving behavior that can lead to endogenous fluctuationsin interest rates. This chapter is based on a joint paper with Eungsik Kim. We analyze the implications of quasi-hyperbolic discounting preference for two types of endogenous economic fluctuations, endogenous deterministic cycles and local sunspot equilibria, ina three-period overlapping generations (OLG) economy with pure exchange. We provide a sufficient condition for the existence of two-period endogenous cycl...