Ancient woodland is a term used in the United Kingdom to refer specifically to woodland that has existed continuously since 1600 or before in England and Wales (or 1750 in Scotland). Before those dates, planting of new woodland was uncommon, so a wood present in 1600 was likely to have developed naturally. The analogous American term is "old growth forest".

For many species of animal and plant, ancient woodland sites provide the sole habitat, and for many others, conditions on these sites are much more suitable than those on other sites. Ancient woodland in the UK, like rainforest in the tropics, is home to rare and threatened species, more than any other UK habitat. For these reasons ancient woodland is often described as an irreplaceable resource, or 'critical natural capital'.

Ancient woodland is formally defined on maps by Natural England and equivalent bodies. Many ancient woodlands have legal protection of various types, but it is not automatically the case that any ancient woodland is protected.

In the Woods is a 2007 mystery novel by Tana French about a pair of Irish detectives and their investigation of the murder of a twelve year old girl. The novel won several awards such as the 2008 Edgar Award for Best First Novel By An American Author, the 2008 Barry Award for Best First Novel, the 2008 Macavity Award for Best First Mystery Novel, and the 2008 Anthony Award for Best First Novel.

Twenty years prior to the novel's events, twelve year-old Adam and his two best friends failed to come home after playing in the familiar woods bordering their Irish housing estate. A search is called by the families, and the police find Adam shivering, clawing the bark of a nearby tree, blood on his shoes and slash marks on his back. He is unable to tell them what happened or where his friends are. His amnesia holds to the present day, where he goes by his middle name, Rob, to avoid the attention of the media to his famous case. His friends were never found.

The plot of the novel circles around the murder of a twelve year old girl, Katy Devlin, whose case Rob and his partner Cassie Maddox are given to investigate. The body is found in the same woods where Rob’s friends disappeared, at an archeological dig site, and the coincidence is enough to make Rob nervous, though he insists to his partner that he is fine.