The political science literature on the subject of transitional justice processes refers to a range of approaches that societies undertake to address legacies of widespread or systematic human rights abuse as they move from a period of violent conflict or oppression towards peace, democracy, the rule of law, and respect for individual and collective rights (ICTJ). Most of the literature on transitional justice mechanisms focuses on trials, truth commissions, reparations and the construction of memorials. Currently, however, there is a dearth of information regarding the transitional justice measure known as vetting. The central purpose of vetting is institutional reform as a means to prevent the recurrence of abuse. Reforms are often aimed at bodies that are responsible for past human rights violations or other forms of repression, such as the military, police or security forces. While trials, reparations and truth commissions tend to be the transitional justice mechanisms most commonly adopted by states, the number of countries that have employed vetting during transitions to democracy is worthy of attention. Thirty-one states, in regions as diverse as South America, Africa and Eastern Europe, have implemented vetting in various forms since World War II. Why has vetting been implemented by some states and not others? This paper seeks to address this question by investigating the causal mechanisms that influence state decisions to implement vetting after a democratic transition. I propose four hypotheses: first, I argue that previous regime type, specifically the nature of regime repression and the type of regime leadership, are important variables that influence whether a state transitioning to democracy will implement vetting. Authoritarian regimes with hierarchically-led militaries tend toward a focus on individual responsibility and typically seek to implement trials when confronting past repression, while authoritarian regimes with non-hierarchically-led militaries and totalitarian regimes tend to focus on collective responsibility which manifests in purges of public institutions. Second, this paper asserts that while historically the most common form of vetting has been purges, in recent decades, increased attention has been given to the importance of incorporating due process in vetting procedures. Many Eastern European states were criticized by human rights organizations and other states for the absence of due process in the lustration (vetting) processes conducted at the end of the Cold War. Since that time, states that have opted to employ vetting have increasingly focused on individual responsibility for human rights violations and ensuring the rights of those targeted by the vetting process. Third, this paper argues that the clustering of vetting states within a similar time frame in Eastern Europe can be explained by Florini's theory of horizontal norm reproduction. Fourth, this paper asserts that international actors, including the UN and non-profit organizations such as the International Center for Transitional Justice (ICTJ), are increasingly playing a role in directing or influencing the transitional justice mechanisms post-conflict states employ. A growing awareness of the need to incorporate due process into vetting in order to consolidate democracy, and the codification of this notion into international law and case law has led international actors to initiate, consult on, and, at times, participate in various vetting processes.The paper is divided into three sections: definitions and explanations of vetting, a discussion of theories concerning democratic transitions and norm emulation and the integration of these theories into an explanation of the vetting cases... ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]