The Amazon Region occupies approximately 7 million square kilometres in the North Central part of South America. Most of it is in Brazil but it also crosses the border of Bolivia, Colombia, Ecuador, Guyana (also a small part of the French Guiana), Suriname and Venezuela. The Amazon represents 58.5 percent of the Brazilian land mass.

The Amazon forest contains the largest single reserve of biological organisms in the world. No one really knows how many species there are in the Amazon forest, but scientists estimate that there are between 800,000 and 5 million species living there, amounting to 15 to 30 percent of all the species in the entire world. As naturalists catalogue new species of freshwater fish, their findings suggest that there may be as many as 3,000 kinds of fish in the Amazon's rivers and lakes.

Among the specialized fish found in the area are: the pirarucu, said to be the largest freshwater fish in the world with specimens measuring over 6.5 feet (2 metres) in length and weighing 275 pounds (125 kilograms); the tambaqui, a member of the fruit-eating characin family which possesses teeth that can crack seeds as hard as those of the rubber tree and the jauari palm; and the piranha. The ferocity of the meat-eating piranha has been exaggerated. Although it is true that some species in rare circumstances have killed large animals and even people, their behaviour depends on the state of their habitat. In main river channels and in larger lakes they appear to leave swimmers unmolested. Only when they lack nourishment do they become aggressive.

Despite centuries of effort to overcome the dominance of nature, people have made little impact on the Amazon and most of its vast drainage basin. No bridge spans the river. Except near its mouth, the Amazon watershed constitutes one of the most thinly populated regions in the world. Much of the territory drained by the river system has never been thoroughly explored. One may fly for hours over the tropical forests that cover much of the river's floodplain and see no sign of human settlement. In many stream valleys, Native American tribes hostile to strangers continue to live much as they did before the arrival of the Europeans.

Most commerce is narrowly confined to the navigable sectors of the river system. The economy continues to be dominated by primitive agriculture, hunting and fishing, and the gathering of various forest products. Commercial farming, tourism, and industry play only a minor role in the region, but mining and lumbering, the principal economic activities, are increasingly important.

In the 1980s Brazil, under pressure from international conservation groups, started to ensure that efforts to develop the Amazon did not irrevocably compromise the nation's forest resources.

The Amazon watershed includes the largest and wettest tropical plain in the world. Heavy rains drench much of the densely forested lowland region throughout the year but especially between January and June. Seasonal variation in rainfall is reflected by the width, rate of flow, and discharge volume of the river.
Because of its vastness, annual floods, and navigability, the Amazon is often called the Ocean River. The total number of its tributaries is as yet uncounted, but more than 200 are in Brazil alone. Seventeen of the largest known tributaries are more than 1600 km (1000 mi) in length. The Amazon proper is navigable to ocean liners of virtually any tonnage for two-thirds of its course.