Compelling and accessible, this Very Short Introduction challenges the perception of borders as passive lines on a map, revealing them instead to be integral forces in the economic, social, political, and environmental processes that shape our lives. Highlighting the historical development and continued relevance of borders, Alexander C. Diener and Joshua Hagen offer a powerful counterpoint to the idea of an imminent borderless world, underscoring the impact borders have on a range of issues, such as economic development, inter- and intra-state conflict, global terrorism, migration, nationalism, international law, environmental sustainability, and natural resource management. Diener and Hagen demonstrate how and why borders have been, are currently, and will undoubtedly remain hot topics across the social sciences and in the global headlines for years to come.