Despite its small size and high development and urbanization, Guam supports a diverse range of terrestrial, freshwater, and marine environments. Some of the key terrestrial environments are different types of forests (limestone forest, ravine forest, scrub forest) and grasslands (savanna vegetation). Due to development, large parts of Guam are now classified as urban vegetation (parks, lawns) and urban builtup areas (buildings, roads, paved places). Southern Guam is dominated by savanna vegetation and ravine forests.

Freshwater environments are also diverse on Guam. Southern Guam has about a hundred named streams, a few ponds and reservoirs, and many wetlands. The wetlands can be divided into freshwater marshes and swamps and estuarine zones and mangroves.

Guam's marine environments are extremely complex. The island is almost completely surrounded by fringing coral reefs. There are also two lagoons, both in southern Guam. Some of the reef flats support sea grass beds, which are under threat by pollution and recreational misuse.