The world is facing an unprecedented loss of biodiversity caused by anthropogenic environmental change. Captive breeding and reintroduction can help mitigate the effects of biodiversity loss for some endangered species, but to accomplish this, captive populations need to be self-sustainable. Intensive population management aims to achieve sustainability by maximising the retention of genetic diversity, maintaining demographic stability, and reducing adaptation to captivity. Recent evaluations of captive populations have indicated that many are not meeting their genetic and demographic goals, and are not sustainable. Consequently, their contribution to biodiversity conservation is being undermined. This thesis aims to evaluate the sustainability of captive populations using the scimitarhorned oryx as a case study. The European scimitar-horned oryx population experiences many of the challenges encountered by other captive populations, specifically, poor data quality in the international studbook resulting in less effective population management; rapid loss of genetic variation; and economic fragmentation. This thesis presents a series of original studies that evaluate the sustainability of captive populations, examines the impact of poor quality data on population management, and tests the effects of population fragmentation. The results contribute knowledge to the management of small captive populations in general, and to the scimitar-horned oryx in particular. I propose solutions to some of the challenges faced by endangered species in captivity, and advocate a reorientation of the existing small population management paradigm. Finally, I challenge the international zoological community to fulfil its potential for biodiversity conservation, and sustainably manage the populations in its care.