The Amazon basin in limited to the west by the Andes, to the north by the Guiana Shield, and to the south by the Mato Grosso plateau. This area of 7 million square kilometers, drained by two of the largest fluvial systems in the world, the Orinoco and the Amazonas, is almost entirely covered by forest. The Andes chain on the west, with an average height of 4000 m and peaks reaching higher than 6000 m, blocks the air that circulates over the Amazon. As the air rises over the eastern slopes the moisture condenses and originates heavy snow and rain fall. Most of the water content of the atmosphere, maybe some 75%, originates from the evapotranspiration of the enormous vegetal biomass of the Amazon forest. Rain and thaw water flow eastwards and feed the great rivers. The Rio Amazonas alone has a mean discharge exceeding 200 000 cubic meters per second at the mouth, representing about 20% of all the fresh water discharged in the ocean by all the rivers in the planet.

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The Amazon forest corresponds to 1/3 of all primitive forests that survived to the present day, and produces 15% of all the oxygen in the Earth's atmosphere. The Amazon forest is probably the most diverse of all Earth's ecosystems. The Amazon is home to 3 000 known species of fishes, 1 500 species of birds, 1 800 species of butterflies, more than 50 000 species of flowering plants. Some estimates point to some 30 million species living actually in the Amazon, for the most part insects, arachnids, millipedes and crustaceans. By contrast, the total number of species presently described from all Earth's ecosystems is only 1.7 million.