A third of all international organisations (IOs) created between 1905 and 2005 no longer exist. Many IOs today are under mounting pressure. Examples include the United Kingdom’s decision to leave the EU, the mooted exodus of African States from the International Criminal Court and the decision of the United States to leave UNESCO, one of the world's best-known global cultural heritage and preservation organisations. Building on existing research about how IOs are designed and develop, the EU-funded NestIOr project will investigate the decline and death of IOs. Specifically, it will study whether larger and more flexible IOs are more likely to live longer because they can adjust, be more responsive to external pressures, and are more difficult to replace.