This paper examines the berserker, a frenzied warrior attested to in both the written and material sources of medieval Scandinavia, and elucidates the characteristics that define him. It critiques explanations for the phenomenon offered in the existing historiography and whether this can be explained as a psychiatric diagnosis. It concludes that the berserker cannot be simply defined as a culturally bound or other psychiatric syndrome, or accounted for by psychogenic drugs alone. Instead, it proposes that berserk frenzy constituted a transitory dissociative state shared among a small warband steeped in religious/spiritual ideology. In entering this state, the psyche of the berserker was reconstituted in an almost archetypal pattern. Further research is required into this phenomenon in other contexts, including modern conflicts.