National security governance has predominantly been organized by the elected branches of government. The executive and legislative branches, many constitutional scholars contend, are institutionally more competent and democratically more legitimate in governing national security matters than courts. However, the conventional view that courts play limited roles in overseeing national security policies is not universally supported, and this is especially the case in ontologically contested democracies, where the boundary of the state or the core interests of national security are inherently contentious due to the prolonged geopolitical tensions these democracies experience. Owing to the distinct nature of ontologically contested democracies, national security governance involves the normalization of geopolitical tensions, uneven public support for the executive’s decisive national security measures, and high-profile national security disputes. These three factors present a particular political and constitutional environment that may encourage courts to oversee national security policies in a rigorous manner. In this dissertation, I examine the oversight of national security policy by constitutional courts in three ontologically contested democracies: South Korea, Taiwan, and Lithuania. Focusing on the behavior of the three constitutional courts, I present three major research findings: First, I show that the methodologies used by the constitutional courts to handle national security disputes have been shaped by their respective international and domestic political contexts. Second, I identify three conditions which contribute to the exercise of oversight by the Constitutional Courts of South Korea, Taiwan, and Lithuania in the national security context: judicial capacity, political schism, and judicial reputation. Regarding judicial capacity, I argue that the three constitutional courts have been familiar with and thus capable of deciding on national security disputes related to civil liberties protections, mandatory military issues, and large-scale military reforms, given that the disputes within these categories have been handled by the courts regularly due to the normalization of national security concerns in these jurisdictions. Regarding political schism, I demonstrate that intense political divisions in South Korea, Taiwan, and Lithuania have given the three constitutional courts substantial leverage to mediate political elites. Finally, concerning judicial reputation, I show that the Constitutional Courts of South Korea, Taiwan, and Lithuania have rigorously exercised oversight in high-profile national security disputes. As these disputes attract widespread attention from domestic or even international audiences, judicial oversight, rather than deference, strengthens the reputation of the courts in the eyes of their target audiences. Third, I demonstrate that by exercising oversight, the Constitutional Courts of South Korea, Taiwan, and Lithuania have stimulated legal reforms, mediated political conflicts, and strengthened the doctrinal significance of national security in their respective jurisdictions. Through these methods, the three constitutional courts have rigorously established the ground rules for formulating and enforcing national security policies. With these findings, I underscore the necessity of carefully considering the nuanced dynamics and effects of the judicial politics surrounding national security law from a comparative perspective. Comparative studies of national security law can broaden and deepen our understanding of the functions of courts within diverse national contexts.