The evolution of the nuclear market explains why there are only nine members of the nuclear club, not twenty-five or more, as some analysts predicted. In the absence of a supplier cartel that can regulate nuclear transfers, the more suppliers there are, the more intense their competition will be, as they vie for market share. This commercial rivalry makes it easier for nuclear technology to spread, because buyers can play suppliers off against each other. The ensuing transfers help countries either acquire nuclear weapons or become hedgers. The great powers (China, Russia, and the United States) seek to thwart proliferation by limiting transfers and putting safeguards on potentially dangerous nuclear technologies. Their success depends on two structural factors: the global distribution of power and the intensity of the security rivalry among them. Thwarters are most likely to stem proliferation when the system is unipolar and least likely when it is multipolar. In bipolarity, their prospects fall somewhere in between. In addition, the more intense the rivalry among the great powers in bipolarity and multipolarity, the less effective they will be at curbing proliferation. Given the potential for intense security rivalry among today's great powers, the shift from unipolarity to multipolarity does not portend well for checking proliferation.