The Comoros archipelago lies in the Indian Ocean east of Mozambique, and is composed of four volcanic islands and the off-shore bank of Geyser and Zelee. Its coastal and marine habitats range from rocky shores to pure white sandy beaches, coral reefs and mangroves. Three islands belong to the Federal Islamic Republic of Comoros, and one is a French Collectivité. Grande Comore and Anjouan are steep-sided, while Moheli has a significant expanse of shallow water. These three are ringed by fringing reefs. Mayotte is a marine paradise with an immense and complicated barrier reef, fringing reefs and many patch reefs, and also has a very uncommon but well studied' double-barrier' reef. More than 80% of the Comorian population live in coastal areas and rely on agriculture and marine resources for their livelihood. Poverty and demographic trends (519,000 in Comoros, 133,000 in Mayotte), include a rate of population increase which is one of the highest in the world. This, and a shift of the economy from primary to secondary activities in Mayotte, create identified threats to the coastal environment. Human pressure on the marine and coastal resources is close to the limits of sustainability . Available data show that anthropogenic threats include both over-harvesting of natural resources and habitat destruction. Marine ecosystems have suffered massively during the last decade from siltation due to poor land use practices and sewage pollution. Mayotte lagoonal communities, with their limited water exchange, are being affected by sediment and pollutant accumulation. Organic and toxic chemicals discharged into the lagoon from various sources over two decades will increase and become more serious over the next decade as well. This may affect, in the medium or long-term, neighbouring marine habitats as well as people living nearby. Coastal and marine resources from the Comoros archipelago are of great importance in the maintenance of biodiversity in this part of the Indian Ocean, but loss of critical habitats such as coral reefs, mangroves and beaches will also be of general significant economic and social concern in the coming decade. Global phenomena such as the El Niño-related massive mortality in 1998 affected this region also. In addition, because of its location beside tanker routes, the Comoros islands remain at risk from oil pollution which, given a tidal range of 4 m, would affect all the coastal ecosystems.