Guadeloupe is composed of two islands: Basse-Terre which is high and volcanic and Grande-Terre which is flat and calcareous. The two islands are separated by a narrow channel which is nowhere more than 200 m wide. Other islands in the archipelago include La Desirade, Marie-Galante and Les Saintes Archipelago plus some small islets.
On the Caribbean coast there are diverse coral communities on the rocky bottoms, particularly around the Pigeon Islets. The Grand Cul-de-Sac Marin in the north is surrounded by a barrier reef, 20 km long, that encloses a 10 km-wide lagoon covering 150 sq km.  The island’s Atlantic coast has fringing reefs, which are more developed on the eastern part of the Basse-Terre (Petit Cul-de-Sac Marin). The surrounding islands are devoid of coral reefs (Les Saintes) or possess narrow fringing reefs (Marie-Galante, La Desirade). 
According to the Reefs at Risk analysis, all the 400 sq km of reefs around Guadeloupe are threatened by human activities. Almost all reefs were rated as threatened by overfishing. Fishing is an important activity in Guadeloupe. In 1998, there were more than 2,000 professional fishermen, with another 1,000 thought to be fishing regularly. Most of the nearshore communities are considered to be overexploited.  Coastal development was identified as a threat to 85 percent of reefs, while sedimentation from the land was identified as threatening about 45 percent of reefs. Threats from anthropogenic sources come from agricultural (banana and sugar cane plantations) pollution, urban pollution, and sediment runoff caused by extensive deforestation for agriculture. About one quarter of reefs were estimated to be threatened by marine-based pollution.
The islands are at risk from hurricanes. Hugo (1989) hit the archipelago head on, while Luis and Marilyn (1995) severely damaged the islands of St. Barth