This dissertation explores the sources of employment protection legislation. Employment protection legislation, or 'EPL,' is regulation which restricts the hiring and firing of workers. I build on and expand the existing literature, which has largely focused on cross-national variation by examining why policies change over time. I focus on the domestic sources of these changes. To do so I develop a theory based on changing population dynamics--specifically, the rise of the boundaryless actor--which result in new preferences over employment protection. These preferences are translated through political institutions--operationalized as veto players--to the outcome arrangements of employment protection.I test this theory, along with competing hypotheses from the existing literature, using both quantitative and qualitative research methods. In addition to a quantitative analysis of 22 OECD countries over 23 years, I present case studies of Australia and New Zealand, utilizing both quantitative and qualitative evidence. There is some evidence supporting the role of boundaryless actors in changing employment protection arrangements. The empirical evidence I present also highlights the role of groups (unions and business associations), veto players, macroeconomic conditions, and legal origins in determining employment protection legislation. Surprisingly, I also find a negative and statistically significant correlation between union density and employment protection legislation in the cross-national data. I attempt to further explore and explain this relationship throughout this dissertation, providing several avenues for future research. In the conclusion I summarize the results and present several implications of this dissertation for future research, policy, and various groups who may wish to affect policy outcomes.