Wingspan 45-75 mm. The males are red-brown with a yellow band across the wing, whereas the females are larger and a paler cream colour. Adults from northern moors and some dunes and southern heaths are often larger and darker than most southern forms.

Frequents woodland edges, hedgerows, downland, fens, sand-dunes, sand-hills and sea cliffs.

Males fly during the day especially in sunshine; females are nocturnal and can be attracted to light. The normal flight period in lowland southern Britain is July to August and in the north from late May to early July.

In the north of Britain development takes two years; in the south one year, with a zone in the Midlands and Welsh borders where it varies, probably with variation in the climate from year to year. The Oak Eggar, despite its name, does not feed on Oak but is so-called because the shape of its cocoon is acorn-like. The foodplants are mainly Heather and Bilberry but also include Bramble, Willows, Broom, Blackthorn, Hawthorn, Hazel and Sea-buckthorn.

Fairly common and widespread in Britain. In a recent survey to determine the status of all macro moths in Britain this species was classified as common.

Fairly frequent but not common in Leicestershire & Rutland. L&R Moth Group status = B (scarce resident or restricted distribution or regular migrant)