An environmental report says that mining, roads and other developments are destroying far too many trees.
The research group which carried out the report is worried that up to 40% of untouched forest land could disappear within 10-20 years.
Some of the forests that are in the most danger are in Russia and Indonesia.
Orang-utans in danger
In Indonesia the loss of forest causes serious problems for animals such as the orang-utan that lives in the trees and is now at risk of become extinct.
The research group says that drastic action must be taken to stop huge areas of forest being completely wiped out.