The western Indian Ocean supports over 6 200 000 pairs of Sooty Terns, some in very large colonies. During the past two centuries colonies have exhibited increase, stability, decline and extinction. The main drivers of these trends have been habitat change and unregulated human exploitation, especially of adults; introduced predators appear to have little effect at the population level but may have prevented re-colonisation following habitat restoration. Regulated harvesting of eggs, based on increasing knowledge of Sooty Tern demography, appears to be sustainable. Some colonies now receive protection but it will be logistically difficult to extend this to all colonies. The main future threats are likely to be climate change and over-exploitation of tuna, on which Sooty Terns depend to feed. Sooty Terns should be monitored to provide insights into these and other perturbations of the marine ecosystem.