Abstract: Over the last thirty years, climate change and depletion of the ozone layer have been widely believed to be the world's largest environmental problems. The two problems have many similarities. Both involve global risks created by diverse nations, and both seem to be best handled through international agreements. In addition, both raise serious issues of intergenerational and international equity. Future generations stand to lose a great deal, whereas the costs of restrictions would be borne in the first instance by the current generation; and while wealthy nations are largely responsible for the current situation, poorer nations are anticipated to be quite vulnerable in the future. But an extraordinarily successful agreement, the Montreal Protocol, has served largely to eliminate the production and use of ozone-depleting chemicals, while the Kyoto Protocol has spurred only modest steps toward stabilizing greenhouse gas emissions. What accounts for the dramatic difference between the two protocols? Part of the explanation lies in the radically different self-interested judgments of the United States; part of the explanation lies in the very different payoff structures of the two agreements. Influenced by the outcome of a purely domestic cost-benefit analysis involving reductions in ozone-depleting chemicals, the United States enthusiastically supported the Montreal Protocol. Influenced by the very different outcome of cost-benefit analyses for reductions in greenhouse gas emissions, the United States aggressively opposed the Kyoto Protocol. An examination of the two protocols suggests that neither agreement fit the simple structure of a prisoner's dilemma, in which a nation gain from an enforceable agreement, gains even more if it is the only nation not to comply while all others do, and lose most if it, and everyone else, pursue their own national self-interest. For the United States, at least, compliance with the Montreal Protocol would have been justified even if no other country had complied; for the United States, and for several other countries, compliance with the Kyoto Protocol would not have been justified even if all other parties had complied. An understanding of the judgments that surround the two protocols indicates that even though moral considerations require the United States to spend a great deal to protect citizens in other nations, and even though such considerations can influence behavior, the nation is unlikely to act in response solely to those considerations. A general implication is that any international agreement to control greenhouse gases is unlikely to be effective unless the United States believes that it has more to gain than to lose. An illuminating wrinkle, also suggestive of the role of domestic self-interest, is that some European nations, above all the United Kingdom, initially contended that ozone depletion was a greatly exaggerated problem while later calling for strong controls on greenhouse gases.