This thesis consists of three essays on economic fluctuations and monetary policy issues, which are investigated within New- Keynesian monetary general equilibrium models. The first essay analyzes the properties of optimal disinflation policies, seeking to identify conditions under which: (i) the optimal disinflationary path involves significant output losses; (ii) a rapid disinflation is preferable to a gradual one. According to our results, different degrees of monetary frictions and inertia in output or inflation may lead to different optimal disinflationary policies - some of which will be quick and painless, while others will proceed slowly and generate deep recessions. The second essay investigates the relationship between credit market imperfections and the degree of shock amplification arising from the so-called financial accelerator, by simulating a macroeconomic model with two types of financial frictions. According to our results, the power of the financial accelerator may either increase or decrease with financial frictions, depending on the source and initial level of such frictions. The third essay provides an empirical investigation of the relationship between credit market imperfections, shock amplification and macroeconomic volatility, based on socioeconomic data from a panel of 62 countries. According to our results, which are consistent with the theoretical model developed in the second essay, macroeconomic volatility seems to increase with contract enforcement costs, but varies non-monotonically with the degree of anti-creditor bias in the judicial and legal system