Berserkers (or Berserks) were Norse warriors who had sworn allegiance to the sky god Odin. They worked themselves into murderous fury before a battle.

The term berserker comes from Norse "berserkr", meaning literally "bear shirt" or "bare shirt", but alluding either to wearing the "clothes" of a bear, i.e. to be bear-like in rage and strength, usually in battle, or to the habit of berserkers going into battle unarmored.

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.

Going berserk – berserksgangr or berserkergang – is described a fit of furry that can happen anytime. It begins with shivering, chattering of the teeth, and a chill in the body. The face swells and changes its color. Next came great rage, howling, and indiscriminate brawling. When the rage quells, the berserker is exhausted and dull of mind for up to several days. According to sagas, many enemies of berserkers exploited this stage to get rid of them.

Today the word "berserker" applies to anyone who fights with reckless abandon and disregard to even his own life, i.e., "goes berserk".