Why have only two of the eleven original members of the Western Climate Initiative implemented a cap-and-trade system? This article compares the implementation of cap-and-trade in California and Quebec versus in New Mexico and British Columbia. Ideas around the reality of anthropogenic global warming and the legitimacy of cap-and-trade created favorable context in three jurisdictions, although institutions condition the expression of these ideas in the policy-making process. Since parliamentary institutions concentrate power, elite consensus is more important in Canada, while in the United States public opinion plays a more significant role. However, ideational factors shaped by political institutions do not explain differences in cap-and-trade implementation. Growth in shale gas production, welcomed in British Columbia and New Mexico but resisted by Quebec and marginal in California, further explain different outcomes. Ideas, mediated by institutions, are the necessary prerequisites for action, while material factors influence policy instrument choice.