Why would any nation choose to share its most vital knowledge openly with others — even with potential adversaries? In the wake of World War II, Danish physicist Niels Bohr urged exactly this in a historic letter to the United Nations. The atomic bomb, with its unprecedented destructive power, had transformed the outlook on international conflict. Bohr argued that only radical openness could contain such a threat and build the trust needed to secure a peaceful future.This volume brings together contributions from leading scholars across disciplines, presented at a Copenhagen conference marking the 75th anniversary of Bohr’s Open Letter. Together, they explore the enduring relevance of Bohr’s call for openness in a world still grappling with global risk, technological upheaval, and the fragile foundations of international cooperation.