A forest is a large area of land covered with a dense growth of trees and other plants. Earth's first forests developed around 365 million years ago. In the early 1800s forests covered 60% of the Earth; today they account for about 30%. Deforestation is a major environmental concern, since forests make such a vital contribution to Earth's atmosphere and also act as a watershed. Forests have been an important source of timber, food, and other resources since prehistoric times.

The forest ecosystem has five basic strata: canopy, understory, shrub layer, herb layer, and forest floor. Forests are classified into three general formations: tropical hardwood forests, including rainforests, are predominantly evergreen. They account for about 7% of the Earth's landmass, but about 50% of the Earth's species. Temperate hardwood forests are mostly deciduous and are found in temperate climates. Boreal forests consisting mainly of conifers, lie in the far north.