To linguists, words and concepts shared by Indo-Europeans suggest that fighting madly was a very old custom that originated perhaps in the fourth millennium B.C. The word for "mad attack," eis-, shared by Vedic, Iranian, and Germanic warriors, makes it likely that the berserk fighting style comes from the time before the dispersal of the Indo-Europeans. Berserks thus embody an abiding spirit in unbroken tradition from Vedic and Homeric times to those of the Icelandic sagas. The history of berserk warriors offers rich religious, cultural, and military detail from about 1300 B.C. to A.D. 1300 and links the bronze, iron, and middle ages, three thousand years of history seldom understood as belonging together. There are also striking similitudes between berserkers and reckless warriors like Aztec quachics and Malabar amoks opening new perspectives for the human knowledge.

The earliest surviving reference to the term berserker is in Haraldskvæði, a skaldic poem written by Thórbiörn Hornklofi in the late ninth century in honour of King Haraldr Fair-Hair, the infamous ruler of Norway. The poem was preserved by Snorri Sturluson. In this poem, Harald's army includes a warrior gang of berserkers fighting under his name at the battle of Hafrsfiord. In it, they are described as Ulfheðnar = "men clad in wolf skins". This grounds a connection between bears and wolves in Norse warrior culture and the common assumption that the word "berserker" itself originates from men wearing the skin of the bear.

Snorri Sturluson goes on to mention berserkers in the Ynglinga saga, written shortly after A.D. 1220:

"his [Odin's] men rushed forward without armor, were as mad as dogs or wolves, bit their shields, and were as strong as bears or wild bulls, and killed people at a blow, but neither fire nor iron told upon themselves" (Ch. 6).

Many sagas describe berserkers as villains who kill, loot, and plunder indiscriminately.
Much can be derived about berserkers from Egils saga. Egil's grandfather was named Kveld-Ulf meaning "evening wolf". Kveld-Ulf's son, referred to as Skalla-Grimm, was a berserker. Kveld-Ulf and Skalla-Grimm are both depicted as irascible and violent throughout the saga. One commits suicide and the latter kills his offspring. Patently, violence and gruesome tragedies permeate the berserker ethos described in Icelandic sagas such as this one.

Erik the Red might have been a berserk. Harald Fair-hair, founder of the kingdom of Norway, used shock troops of berserker warriors. Warriors of the Varangian Guard (Norse warriors working for Byzantine Empire) also followed bear rituals. Many northern kings used berserkers as part of their army of hirthmen and sometimes equivalent to a royal bodyguard.

Berserkers fought with crazed or drugged strength, heedless of danger. They worked themselves up into a bloodlust – berserker rage – before battles, banging their helmets with their weapons, biting their shields, and howling. They were said to be immune to pain (or even immune to weapons) in battle. In their fury they would attack their enemies but also everything else in their path, sometimes even their own people and allies.

Six-hundred years after Beowulf, Saxo Grammaticus in his early thirteenth-century "Gesta Danorum" says that Asmund flung his shield on his back to fight more fiercely and daringly and hence win greater fame. Norway's King Hákon the Good in 935 and in 961 also trod the battlefield as an armor-scorning fighter:

He threw off his armor
thrust down his mail-coat
the great-hearted lord,
ere the battle began.
He laughed with his liege-men.

Hákon's laughter showed his scorn of wounds. Such berserk-gestures by individuals, often kings and other leaders of men, abound in Nordic warrior tales.

Medieval berserks were often battle lords. In the tenth-century battle on the Vín Heath in Northumbria, Thorolf, the Icelandic Viking wore a helmet but no hauberk, and when the battle went badly, he "became so berserk that he swung his shield round to his back, and took his spear in both hands. He ran forward, striking or thrusting on both sides. Men sprang away in all directions, but he killed many.... Then Thorolf drew his sword, striking out on both sides, and his men also joined the attack."(n79) Flinging one's shield to one's back as a berserk gesture is found on reliefs of the berserk-like Shardana guard of Ramses II and on archaic Greek warrior statuettes.(n80)

Icelandic sagas often tell of berserks as wild, howling fighters, sometimes as high-born champions of kings, sometimes as lowly drifters.

Allies to the raging Norsemen were wary of berserkers. Fearing that their own homesteads and families might be targeted by the berserkers' violent instability, friendly Norsemen kept women and children at bay.

In 1015 King Eirik Bloodaxe (Eric I) of Norway outlawed berserkers. Grágás, the medieval Icelandic law-code, sentences berserker warriors to outlawry. By the 1100s organized berserker warbands had disappeared.

Christianity forbade berserks, but their spirit lived on. Among island Celts it survived longest. Pawns of the twelfth-century chess set from the Isle of Lewis in the Outer Hebrides are portrayed as warriors who bite their shields in battle madness.

Also, when in 1138 King David of Scotland met an Anglo-Norman army in the Battle of the Standard, his Galwegian and Highlander warriors claimed their right to attack ahead of his armored household knights. With lances and swords they ran into battle unarmored, full of fury and daring, only to be shot down by English bowmen. The few who reached the English line achieved nothing against the armored, dismounted knights who led the defense. When they fled, they dragged the rest of the Scottish army into a rout, just as did the naked Celtic Gaesati at Telamon 1350 years earlier.

Modern age

The word "berserker" today applies to anyone who fights with reckless abandon and disregard to even his own life, a concept used ad nauseam during the Vietnam War and in Vietnam-inspired literature (Michael Herr's Dispatches) and film (Oliver Stone's Platoon).

"Going berserk" in this context refers to an overdose of adrenaline induced opioids in the human body and brain leading a soldier to fight with raging fearlessness and indifference, a state strikingly similar to that of the 9th century berserkers observed in this article. "Going berserk" is also used colloquially to describe a person who is acting in a wild rage or in an uncontrolled and irrational manner.