This chapter presents the results of a global quantitative analysis designed to test three theoretical explanations for the worldwide expansion of constitutional environmental rights—domestic politics, norm socialization, and rationalism–materialism. Scholars have offered many potential explanations for the trend toward constitutionalization of environmental rights that began in the 1970s. Other hypotheses refer more generally to the pursuit of legitimacy in the eyes of the international community. Some researchers suggest that the global expansion of environmental rights represents a logical outgrowth of the increased international concern about the state of the environment beginning in the 1960s. Gutmann presents an international, quantitative examination of the expansion of constitutional environmental rights. The chapter discusses that constitutional environmental rights are more likely to emerge in countries with a greater presence of international civil society organizations, a poorer record of human rights practices, and higher levels of democratic governance.