This introduction presents an overview of the key concepts covered in the subsequent chapters of this book. The book focuses on determining the factors that have contributed to the global emergence of environmental rights in national constitutions. It traces the history of environmental rights as they developed through human rights institutions at the international level. The book describes the development of constitutional environmental rights at the national level. It seeks to explain the diffusion of constitutional environmental rights by drawing on insights from international relations (IR) theory, world society theory, and constitutional design theory. The book reports the findings of a global statistical analysis that represents the most comprehensive and theoretically driven attempt to analyze the emergence of constitutional environmental rights to date. It uses the results of the quantitative analysis to identify a pair of countries, Nepal and Sri Lanka, that share characteristics determined to affect the likelihood of adopting environmental rights, yet experience different constitutional outcomes.