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The Roseate Tern, Sterna dougallii, is currently regarded as one of the most endangered seabirds in Europe. The mid-Atlantic Azores islands, Portugal, are an important breeding ground for this species, holding more than 50 per cent of the European population. EU directives are aimed at maintaining and enhancing the current range and breeding numbers of Roseate Tern in the Azores and elsewhere and it is thus a priority species on the Conservation Agenda. However, few studies have examined the Roseate Tern in this geographical area of the Atlantic, and this thesis aims to address this need. The thesis investigates and examines several potential factors affecting breeding numbers of these birds in the archipelago, giving particular attention to predation and potential predators. Data were mainly attained through fieldwork conducted in the period 2002-2004, but investigation of recent historical records of breeding numbers is utilized to assess trends. These historical data were collected between 1989 and 2005, and show that the breeding population has fluctuated widely over the last 16 years. Taking the Roseate Tern demographic parameters into account, this variation can only be explained by means of intermittent breeding. Many more years of research and monitoring will be needed before we can fully understand the factors underlying the decision of Roseate Terns to breed or not in a given year in the archipelago. Nevertheless this lack of a complete understanding of a perhaps natural variation does not prevent implementation of immediate conservation action. This thesis examines the impact of avian predators, such as gulls and starlings, on breeding success. Although terns and starlings can nest in close proximity in the Azores, egg predation by starlings has rarely been reported. In the Azores, this behaviour has only been detected in one colony, Vila islet that holds about 20 per cent of the Azorean Roseate Tern population, but it is the main cause of egg losses in the tern colony. A census conducted to estimate the size of the Yellow-legged Gull population yielded an estimate of 4249 breeding pairs, representing an increase of almost 60 per cent from the previous survey conducted in 1984. A control taste aversion (CTA) experiment was conducted to explore the possibility of using this technique as a way to reduce starling and gull predation. Our results indicate that CTA has potential to work with gulls but it will be much more difficult to use it successfully on starlings. Using an integrated habitat management strategy at Praia Islet, Graciosa, I demonstrate that it is possible to enhance the habitat of the terns and increase their breeding numbers if efforts are global and broad. Future conservation action might need to implement lethal control of starlings in very specific situations. Lethal control should only be undertaken in conjunction with other measures, such as the provision of Roseate Tern artificial nests and close monitoring to assess its effect on productivity. The thesis concludes by summarizing the management and conservation implications of the results and placing these ornithological findings in a wider conservation perspective. By rapidly implementing the actions here described, the populations of Roseate Terns should increase in future to the point where it is no longer critically endangered.