This book provides a comprehensive understanding of environmental regionalism at the international level, analyzing the concept and identifying recurring patterns from six in-depth case studies. While ecoregions or environmental regions are defined on ecological boundaries rather than administrative criteria, ecoregionalism is the idea that regional dynamics should cluster around ecoregions, while ecoregionalization is the tendency of regional dynamics to cluster around ecoregions. Focusing on the international level, this book presents six cases of ecoregional processes from around the world and the regional environmental agreements: two are terrestrial, the Alps and the Andes; two are marine, the Mediterranean Sea and the Baltic Sea; two are related to freshwater ecosystems: the Amu Darya in Central Asia and the Great Lakes in North America. The book analyzes both ecoregional processes focused on the environment, as well as intersectoral ecoregional processes. The case studies are analyzed based on the ecoregional governance framework, developed by the author for this book. Despite the diversity of context, the similarity of the governance system of the six cases is striking. Several recurring patterns have been identified, which may also extend to the subnational level. They are not design principles, but may be taken into consideration for the design or redesign of current and future regional environmental agreements and processes. This book will be of great interest to students and scholars of environmental politics, natural resource management, spatial planning and international relations.