This dissertation encompasses three related essays on US trade unions. Chapter 1 investigates union density differences across states and state-industry in the private sector. In the US, private sector union density varied sharply geographically. This essay examines the factors that contributed to the decline in unionization by exploiting variations at the state and state-industry level. It updates the literature that focused on the pre-1980 period, develops three new measures to gauge the effects of union activism and management opposition on unionization, and uses Oaxaca-Blinder decomposition to identify the major contributors to declining unionization rates. Ordinary Least Squares and Two-Stage Least Squares estimations are carried out for 1985, 1995, and 2005. State-industry estimation results show that union density varied directly with earnings, share of blue-collar workers, union activism, and urbanization rate, and inversely with female share, employer opposition, and Right-to-Work laws. Overall, our model confirms the findings of the previous studies and it verifies our choice of proxies for employer opposition and union activism. Chapter 2 investigates union density differences between Colorado and Utah in the public sector. Colorado and Utah experienced very different histories of change in union density during 1983-2008, despite their substantial similarities with respect to the determinants of unionization. This essay took a comparative analysis approach, because some of the phenomena that affect unionization are the state-specific characteristics and are hard to quantify. The decline in public sector union density in Utah was due to changes in public attitudes, which led to a super majority control of government by the Republican Party and management’s resistance towards unions. Chapter 3 investigates the National Labor Relations Board member’s voting behavior on Unfair Labor Practice cases. Empirical studies show that the board members of the Democratic Party are more likely to vote pro-union and members of the Republican Party are more inclined to vote promanagement. This chapter utilized logistic regression and estimated factors that influence board members’ voting behavior during the 1993-2008 period. Our results confirm the findings of previous studies on board members’ voting behavior and, furthermore, they show that a newly introduced board member’s background variable has an immense impact on a board member’s voting behavior.