Ancient woodland is defined as any woodland which has remained woodland for the last 400 years (since 1600) or more in England, Wales and Northern Ireland (1750 in Scotland).
It can either be Ancient semi-natural woodland (ASNWs), plantation on ancient woodland site (PAWS), Restored Ancient Woodland Site (RAWS) or Ancient Woodland Site of unknown category (AWSU).
ASNWs refer to ancient woodlands that retain a native tree and shrub cover that has not been planted, although it may have been coppiced or felled and allowed to regenerate naturally. PAWs are ancient woodlands where the original tree cover has been felled and replaced by planting, often with conifers , usually over the last century and are comprised of a canopy cover of more than 50% non-native conifer tree species. RAWs are ancient woodlands that will have gone through a phase when canopy cover will have been more than 50% non-native conifer tree species and now have a canopy cover of more than 50% broadleaf. AWSUs are woodlands which may be any of the above and are predominantly in transition where the existing tree cover is described as shrubs, young trees, felled or ground prepared for planting.
Ancient woodlands are particularly important because they are typically exceptionally rich in wildlife including many rare species and habitat and act as reservoirs from which wildlife can spread into new woodland. They preserve the integrity of soil ecological processes and associated biodiversity. They are also an integral part of Britain’s historic landscape having been managed by traditional methods for centuries and demonstrate an enduring relationship between people and nature. A starting point for identifying ancient woodland is the ancient woodland inventory (AWI) which is a list, by county, of sites greater than 2ha that fulfil the aforementioned criteria. These inventories, now brought together as the national Ancient Woodland Inventory, have become an important tool for policy makers and planners whilst also assisting land managers to identify key areas for the restoration and planting of native woodlands and increasing awareness of the importance of ancient woodlands. This is not regarded as definitive but rather provisional as at any stage new information may become available that shows that woodlands not currently on the list are likely to be ancient or vice versa.
Where a woodland is below 2ha or is suspected to be ancient and not included in the inventory, evidence from various records can be used. This may include historical documents such as estate records and enclosure surveys, wood names reflecting nearby settlements, or old words relating to woods, and their location towards the parish boundaries, on steep slopes or valley sides, or generally unsuitable agricultural ground.
To further clarify a woodlands status field surveys can be carried out which may reveal the presence of old/large coppice stools, or veteran trees, well-developed boundary banks and ditches amd the presence of ancient woodland indicators species, particularly vascular plants.

The list of 100 species of Ancient woodland vascular plants (AWVP) was compiled for this purpose by the former Nature Conservancy Council (NCC) in 1978 for South England regions which followed with lists for other regions and subsequent revisions. The list included species characteristic of ancient woodland that are slow to disperse and do not colonise new areas easily.

This relatively short list of species enables rapid assessment in the field and one site to be compared more easily to another. Moreover one can be more confident at finding more of the species present in a short list that those on a long one.