In the United Kingdom, ancient woodland is that which has existed continuously since 1600 in England, Wales and Northern Ireland (or 1750 in Scotland). The practice of planting woodland was uncommon before those dates, so a wood present in 1600 is likely to have developed naturally.

In most ancient woods, the trees and shrubs have been felled periodically as part of the management cycle. Providing that the area has remained as woodland, the stand is still considered ancient. Since it may have been cut over many times in the past, ancient woodland does not necessarily contain trees that are particularly old.

For many animal and plant species, ancient woodland sites provide the sole habitat. Furthermore, for many others, the conditions prevailing on these sites are much more suitable than those on other sites. Ancient woodland in the UK, like rainforest in the tropics, serves as a refuge for rare and endangered species. Consequently, ancient woodlands are frequently described as an irreplaceable resource, or 'critical natural capital'. The analogous term used in the United States, Canada and Australia (for woodlands that do contain very old trees) is "old-growth forest".

Ancient woodland is formally defined on maps by Natural England and equivalent bodies. Mapping of ancient woodland has been undertaken in different ways and at different times, resulting in a variable quality and availability of data across regions, although there are some efforts to standardise and update it.

A variety of indirect legal protections exist for many ancient woodlands, but it is not automatically the case that any given ancient woodland is protected. Some examples of ancient woodland are nationally or locally designated, for example as Sites of Special Scientific Interest. Others lack such designations.

Special consideration is also required for Ancient woodlands when they are affected by planning applications. The National Planning Policy Framework, published in 2012, is the British government's policy document relating to planning decisions affecting ancient woodlands. The irreplaceable nature of ancient woodlands is elucidated in paragraph 118 of the NPPF, which states: ‘Planning permission should be refused for development resulting in the loss or deterioration of irreplaceable habitats, including ancient woodland and the loss of aged or veteran trees found outside ancient woodland, unless the need for, and benefits of, the development in that location clearly outweigh the loss.’

The concept of ancient woodland, characterised by high plant diversity and managed through traditional practices, was developed by the ecologist Oliver Rackham in his 1980 book Ancient Woodland, its History, Vegetation and Uses in England, which he wrote following his earlier research on Hayley Wood in Cambridgeshire.

The definition of ancient woodland includes two sub-types: Ancient semi-natural woodland (ASNW) and Planted ancient woodland site (PAWS).