This dissertation consists of three chapters. The first chapter concerns the secular changes in the U.S. firm size distribution and firm dynamics. This chapter sets up a quantitative model of firm dynamics with debt heterogeneity to study the implications of changes in real interest rates for the firm size distribution and firm dynamics. It shows that the decline in long-term real interest rates since the early 1980s can account for a significant fraction of the shift in employment shares to large firms as well as the decline in firms per capita and firm entry rates experienced in the U.S. over the same period. In the model, firms endogenously choose financial intermediaries issuing debt with either earnings-based (EBC) or asset-based (ABC) borrowing constraints. The two types of constraints arise naturally from the imperfect enforceability of debt contracts and are in line with recent empirical findings. A decline in real interest rates benefits firms with EBC more because they are not constrained by their assets and can expand more due to increased earnings. Since firms with higher earnings optimally choose earnings-based lending, the decline in real interest rates shifts employment shares to larger firms. Moreover, the growth of large firms crowds out smaller firms and firm entry through general equilibrium effects. The paper tests the mechanism in cross-country data from the OECD and finds a stronger association between the decline in real interest rates and changes in firm dynamics, especially in countries with deeper credit markets. In the second chapter, I study the effects of government regulations on firm dynamism. The impact of government regulations on the economy is a central topic in policy debates. However, due to the endogeneity of regulations and challenges in measuring them, these debates remain contentious. This paper establishes the causal effects of government regulations on firm dynamism by employing a novel shift-share (Bartik) instrument in conjunction with the RegData dataset, which quantifies regulations based on the text of federal regulatory documents. The primary assumption for identification is that, for each sector, the exposure to regulations from different government agencies at the beginning of the period is exogenous to any confounding factors. The findings reveal that government regulatory restrictions significantly increase firm exit rates and discourage the formation of establishments, while having no substantial impact on firm entry. Furthermore, these restrictions contribute to reduced job creation, elevated job destruction, and diminished overall employment. These effects are consistently observed across various age groups. The results lend support to the idea that government regulations can raise production costs for firms and/or enhance the monopolistic power of certain companies. Both mechanisms can diminish the profits of affected firms, leading to increased firm exit rates and reduced labor demand. Additionally, the findings refute the interpretation of regulations as solely serving as entry barriers. The final chapter of the dissertation investigates the labor market outcomes for involuntary part-time workers and their subsequent effects on welfare levels. Through an analysis of survey data, I demonstrate that involuntary part-time workers exhibit reservation wages comparable to those of unemployed workers. This similarity largely stems from parallel wage offers and offer arrival rates. Contrary to previous research, this finding indicates that involuntary part-time workers experience welfare levels akin to unemployed workers. One possible explanation for this discrepancy lies in the methodology of prior studies. Conclusions drawn from earlier research, which primarily focused on the faster transition of involuntary part-time workers into full-time positions compared to other workers, may be flawed. This is because these workers also tend to revert to their previous job types at a faster rate. To further explore the implications of these discoveries, I employ a quantitative search model. The calibrated model supports the assertion that involuntary part-time workers experience welfare levels similar to those of unemployed workers. Furthermore, the model suggests that neither extending unemployment insurance to part-time workers nor enhancing the likelihood that unemployed workers transition to part-time positions would effectively increase the prevalence of full-time employment