In the face of daunting environmental challenges, leadership in the development of environmental policy may be more important now than ever. While some countries are willing to devote substantial time and resources to working on environmental issues, others lag behind. In this dissertation, I explore what it means to be a policy leader, the state and trajectory of environmental policy leadership globally, and which actors systematically catalyze (or obstruct) environmental policy leadership. I propose that policy leadership is an observable, persistent behavior in which a government is 1) innovative, adopting new policies quickly and 2) influential, inspiring other governments to adopt these same policies. I argue that transnational advocates, such as international NGOs and multinational corporations, are particularly well-positioned to strengthen (or weaken) policy leadership. Providing information, building capacity, and conducting pressure campaigns in multiple jurisdictions at the same time, transnational advocates are uniquely positioned to affect both the spread and uptake of new policies. Using novel data on the adoption of 3,000 environmental policies by 185 countries, I measure leadership in the development of environmental policy over time and assess the role of transnational advocates in policy development. I find that large international environmental NGOs spark environmental policy leadership, particularly in developing countries and on issues related to traditional environmental topics of flora, fauna, and pollution. In contrast, lobbying by multinational corporations in developed countries sharply reduces environmental policy leadership. These results demonstrate the substantial and varied impacts of transnational advocacy on environmental policy leadership. I make three notable contributions in this dissertation. First, I improve on prior conceptualizations of policy leadership, providing a definition that is straightforward to operationalize while speaking to more general understandings of leadership. Second, I develop a novel method for identifying policy adoptions from a compilation of laws and regulations; this generates a new database of environmental policy adoptions that overcomes the geographic and topical limitations of existing datasets. Finally, I contribute the first systematic, quantitative evidence of the impact of transnational advocates on policy change, demonstrating the importance of their activities for environmental policy development. With little time to waste, it is critical to understand why and when governments act as environmental policy leaders, and what can be done to facilitate environmental policy leadership. Through this research, it is clear that accounting for the activities of transnational advocates should be a key component of any attempt to catalyze proactive and meaningful environmental policy change.