According to legend, he roamed the Fens, which covers parts of the modern counties of Cambridgeshire, Lincolnshire and Norfolk, and led popular opposition to William the Conqueror.

This probably indicates, as the preface to the Gesta suggests, that conflicting oral traditions about Hereward were already current in the Fens in the late 11th and early 12th centuries.

In addition, there may be some partisan bias in the early writers: the notice of Hereward in the Peterborough Chronicle, for instance, was written in a monastery, which he was said to have sacked, some fifty years after the date of the raid.

[4] On the other hand, the original version of the Gesta was written in explicit praise of Hereward;[5] much of its information was provided by men who knew him personally, and principally, if the preface is to be believed, a former colleague-in-arms and member of his father's former household named Leofric the Deacon.

[7] The 12th-century Latin text purports to be a translation of an earlier (and now lost) work in Old English, with gaps in the damaged original filled out from oral history.

The prologue also reports that the earlier, Old English version was badly damaged but not destroyed: the author of the Gesta Herewardi had been instructed by his superior to seek out the remains of Leofric's work and to translate it into Latin.

[9] According to the historian Janet D. Martin, the book was created in "about 1250", and originally ended with the Gesta Herewardi, but further material, unrelated to the Hereward story, was added in the 14th century.

The earliest references to his parentage, in the Gesta, make him the son of Edith, a descendant of Oslac of York, and Leofric of Bourne, nephew of Ralph the Staller.

[15] Peter Rex, in his 2005 biography of Hereward, points out that the campaigns in which he is reported to have fought in the region of Flanders seem to have begun around 1063 and suggests that, if he was 18 at the time of his exile, he was born in 1044/45.

The Domesday Book shows that a man named Hereward held lands in the parishes of Witham on the Hill and Barholm with Stow in the southwestern corner of Lincolnshire as a tenant of Peterborough Abbey;[17] prior to his exile, Hereward had also held lands as a tenant of Croyland Abbey at Crowland, 8 miles (13 km) east of Market Deeping in the neighbouring fenland.

According to the Gesta Herewardi, Hereward was exiled at the age of eighteen for disobedience to his father and disruptive behaviour, which caused problems among the local community.

Historian Elizabeth van Houts considers this aspect of the story to be consistent with evidence concerning expeditions led by Robert the Frisian on behalf of his father Baldwin V, Count of Flanders in the early 1060s.

At the time of the Norman conquest of England, he was still in exile in Europe, working as a successful mercenary for Baldwin V. According to the Gesta he took part in tournaments in Cambrai.

In 1071, Hereward and Morcar were forced to retreat to their stronghold and made a desperate stand on the Isle of Ely against the Conqueror's rule.

Both the Gesta Herewardi and the Liber Eliensis claim that the Normans made a frontal assault, aided by a huge, mile-long timber causeway, but that this sank under the weight of armour and horses.

It is said that the Normans, probably led by one of William's knights named Belasius (Belsar), then bribed the monks of the island to reveal a safe route across the marshes, resulting in Ely's capture.

[23] An ancient earthwork about 1.2 miles (2 km) east of Willingham, Cambridgeshire is still visible at the junction of the old fen causeway and Iram Drove.

This circular feature, known as Belsar's Hill,[25] is a potential site for a fort, built by William, from which to attack Ely and Hereward.

Hereward's former gaoler persuaded the king to negotiate once more, and he was eventually pardoned by William and lived the rest of his life in relative peace.

[23] Geoffrey Gaimar, in his Estoire des Engleis, says instead that Hereward lived for some time as an outlaw in the Fens, but that as he was on the verge of making peace with William, he was set upon and killed by a group of Norman knights.

Hugh M. Thomas argues that the Gesta is intended to be an entertaining story about an Anglo-Saxon hero creating a fantasy of successful resistance to the Normans.

By the 15th century, the Wake family were claiming descent from him and elevating his ancestry by asserting that he was the son of Leofric, Earl of Mercia and Lady Godiva.

It drew on the theory that traditional English liberties were destroyed by the "Norman yoke", an idea earlier popularised in Walter Scott's novel Ivanhoe.