Frontier forests are the world's remaining large intact natural forest ecosystems. These forests are -- on the whole -- relatively undisturbed and big enough to maintain all of their biodiversity, including viable populations of the wide-ranging species associated with each forest type. As defined in this assessment, a frontier forest must meet seven criteria:

1. It is primarily forested.
2. It is big enough to support viable populations of all indigenous species associated with that forest type -- measured by the forest's ability to support wide-ranging animal species (such as elephants, harpy eagles, or brown bears).
3. It is large enough to keep these species' populations viable even in the face of the natural disasters -- such as hurricanes, fires, and pest or disease outbreaks -- that might occur there in a century.
4. Its structure and composition are determined mainly by natural events, though limited human disturbance by traditional activities of the sort that have shaped forests for thousands of years -- such as low-density shifting cultivation -- is acceptable. As such, it remains relatively unmanaged by humans, and natural disturbances (such as fire) are permitted to shape much of the forest.
5. In forests where patches of trees of different ages would naturally occur, the landscape exhibits this type of heterogeneity.
6. It is dominated by indigenous tree species.
7. It is home to most, if not all, of the other plant and animal species that typically live in this type of forest.

Threatened frontier forests are forests where ongoing or planned human activities (such as logging,agricultural clearing, and mining) will eventually degrade the ecosystem (through, for example, declines in or local extinctions of plants and animals or large-scale changes in the forest's age and structure).

Low-threat potentially vulnerable frontier forests are those not now considered under enough pressure to degrade ecosystems. But because they are unprotected and contain valuable natural resources, or because human encroachment is likely, most of these forests are vulnerable to future degradation and destruction.

Non-frontier forests are dominated by secondary forests, plantations, degraded forest, and patches of primary forest that do not meet this study's frontier criteria (even though some might be restored as frontier forest). This category includes some of the world's most unique, valuable, and endangered forest types, including the biologically rich, highly fragmented forests of Madagascar and Central Europe's last stands of old-growth forest. Such forests are high priorities for conservation. Non-frontier forests are also important because they provide us with a wide range of economic goods and services.

Original forest is that estimated to have covered the planet about 8,000 years ago, before large-scale disturbance by modern society began.

A note about the data: This study's results represent estimates of the extent and location of frontier forests as well as threats to their survival. Incomplete information from some parts of the world and difficulties estimating the magnitude of threats make the data suitable only for regional comparisons and for distinguishing major differences among countries' forests. This study did not assess woodland areas of the world, or forests within many island countries. (See Technical Annex).