The efforts of states to provide security for themselves by cooperating to avoid internecine war, to deter and to defeat common foes, and to rein in the proliferation of dangerous weapons, have led to the creation of numerous international security institutions (ISIs) since the emergence of a globe-spanning system of sovereign states in the 19th century. Yet, some ISIs have been more effective than others in adapting to the changes in the international balance of power during the last two centuries. I define effectiveness as member state cooperation to solve common security challenges. At a minimum, states belonging to an effective ISI must not wage war against each other or actively seek to thwart their common efforts to address a security problem. The post-1815 Concert of Europe was relatively effective for more than three decades until the outbreak of the Crimean War in 1854. The Nuclear Non-Proliferation Treaty (NPT) has remained effective despite unsettling shifts in the international balance of power since it was first negotiated in 1968. The NATO alliance has survived the end of the Cold War, arguably the most significant power shift in the last century. What explains the variation in how security institutions respond to power shifts? What are the factors that determine whether an ISI will remain effective through major structural changes in the international system? Extant works in the literature either claim that institutional effectiveness declines post-power shift; that post-hegemonic cooperation is possible; or that the structure of ideas takes precedence over the distribution of material power, thus denying that a material power shift should necessarily have any influence on ISI effectiveness. In contrast, I argue that the interaction of two variables best explains how a significant power shift will affect an ISI’s effectiveness: the number of great powers in the institution and the degree to which the rules are mainly directed at constraining minor power behavior. An increase in the number of great powers in an ISI aggravates bargaining and coordination problems, while rules written in a previous era that apply to all states will be challenged by great powers that have gained relative power. Great powers are especially likely to challenge existing rules that do not reflect the new balance of power because, by definition, they possess the capabilities to put up a serious fight against even the strongest state in the international system. Such states do not meekly respect the status quo when they have gained relative power: they will demand a rewriting of the rules to better reflect their new power advantages and if their demands are not met, they will flout the old rules to the extent made possible by their improved position in the balance of power, challenging other states with an interest in maintaining the status quo. Thus, the most effective ISI will have only one great power with rules that mainly constrain minor powers while leaving the great power unconstrained. I conduct case studies on the Concert of Europe, the NPT, and NATO to demonstrate the plausibility of my approach. First, the effectiveness of the Concert of Europe was vulnerable to a significant power shift because its rules sought to control the behavior of great and minor powers alike, and it included all the great powers in the system, complicating inter-state bargaining and coordination. The rise of industrial Britain towards the middle of the 19th century and the lack of common interest binding Russia and the Western powers led to the Crimean War and the total breakdown of the Concert. Meanwhile, the NPT, despite some great power coordination problems, still managed to remain effective after a major structural change such as the end of the Cold War, because its rules mainly focus on preventing minor powers from acquiring nuclear weapons. Finally, NATO has remained effective because it has only one great power, the United States, which provides security to dependent allies. NATO is a nuclear alliance with the US providing most of the extended deterrence, implying that minor power members have an obligation to refrain from acquiring nuclear weapons, an obligation which does not constrain the US which is doubly recognized as a legitimate nuclear power by the NPT and NATO.