This dissertation examines why and how states pursue multilateral agreements to govern weapons, and how their purpose in doing so shapes the process and outcome of agreements. Weapons governance is a particularly challenging form of international cooperation, as it requires countries to place limits on the tools they use to ensure their security. This research emphasizes the importance of multilateral governance in sustaining or challenging relations between states in world politics. In this dissertation, I make a distinction between two types of states—great powers and small/medium states—and contend that they pursue multilateral weapons governance for different geopolitical purposes, given their different positions and relations to each another in world politics. Specifically, great powers pursue multilateral weapons governance to preserve their status quo dominance and to prevent other actors from threatening their dominance or contesting the status quo. Small and medium states, on the other hand, pursue multilateral weapons governance to reduce their vulnerability to great powers and to exercise greater agency and influence in international politics. These distinct purposes in turn shape how states pursue an agreement, including how they frame the issue, build multilateral support, the institutional format they choose for negotiations, and the content of the agreement. When both types of states seek to govern a particular type of weapon, the tension and conflict between their objectives limits each’s ability to translate their objectives into a final agreement. To analyze these dynamics, I draw on elite interviews conducted in Geneva, Switzerland with diplomats, international bureaucrats, and members of civil society; multiple archival sources; and an original dataset I developed of multilateral weapons governance agreements. Using descriptive statistics and quantitative text analysis, I first assess patterns in the content of agreements including the type of weapon governed, the type of regulation involved, and the legal status of the agreement. I then compare three case studies: the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (led by great powers), the 2017 Treaty on the Prohibition of Nuclear Weapons (led by small and medium states), and the 1980 Convention on Certain Conventional Weapons (as a middle case).