The United States, the world's largest emitter of greenhouse gases, is not going to ratify the Kyoto Protocol in the foreseeable future. Yet, a number of countries have decided to stay on the Kyoto track. Four main explanations for this apparent puzzle are considered. The first is that remaining Annex I countries still expect the Kyoto Protocol to reduce global warming sufficiently to outweigh the economic costs of implementation. The second is that the parties, by implementing the treaty, hope to induce non-parties to follow suit at some later stage. A third hypothesis is that EU climate institutions have generated a momentum that has made a change of course difficult. Finally, Kyoto's persistence may be linked to the European Union's desire to stand forth as an international leader in the field of climate politics. We conclude that the first two explanations have little explanatory power, but find the latter two more promising.