The Berserkers are a race of machines left over from an interstellar war between two alien races. Unlike the benign probe concept, Berserkers are programmed to seek out and exterminate lifeforms and life-bearing exoplanets whenever they are encountered.

These Berserkers, named after the human berserker warriors of Norse legend, are doomsday weapons left over from an interstellar war between two races of extraterrestrials some 50,000 years ago. They all have machine intelligence, and their sizes range from that of an asteroid, in the case of an automated repair and construction base, down to human size (and shape) or smaller.

Berserkers are self-replicating war machines programmed with one main objective: Destroy all life. After destroying both their creators and the opposing side in a long-ago galactic war, the self-replicating Berserkers have continued to wipe out all forms of life that they encounter in the Milky Way, which leads to the cooperation and coordination of most of the sentient races in major attempts to defeat them. Humankind, although relatively new to the galactic scene, is a major player because of its aggressive nature. The series spans a large range of both time and space, and so has less plot continuity than Saberhagen's other series.

The Berserkers' bases are capable of manufacturing more and deadlier Berserkers as their perceived "need" arises. The original Berserkers had been designed and built as an Ultimate Weapon to kill the enemy in the long-ago war. The race which created them somehow forgot how to turn them off, and it was killed off itself. Then Berserkers set about killing all sentient life wherever it could be found. When the Berserkers found out how deadly some forms of life could be to them, some were constructed to wreck entire life-bearing planets and moons, against the possibility that their life might someday evolve into sentient life.

The name is derived from a series of novels by Fred Saberhagen which feature an ongoing war between humanity and such machines (see: Berserker). Saberhagen points out (through one of his characters) that the Berserker warships in his novels are not von Neumann machines themselves, but the larger complex of Berserker machines — including automated shipyards — do constitute a von Neumann machine. This again brings up the concept of an ecology of von Neumann machines, or even a von Neumann hive entity.

It is speculated that Berserkers could be created and launched by a xenophobic civilization (see Anvil of Stars, by Greg Bear, in Examples in fiction below) or could theoretically "mutate" from a more benign probe. For instance, a von Neumann ship designed for terraforming processes — mining a planet's surface and adjusting its atmosphere to more human-friendly conditions — might malfunction and attack inhabited planets, killing their inhabitants in the process of changing the planetary environment, and then self-replicating and dispatching more ships to attack other planets.

The Berserker stories (published as novels and short stories) describe humanity's fight against the Berserkers. The term "humanity" refers to all sentient life in the Milky Way Galaxy, emphasizing the common threat the Berserkers pose toward all forms of life. Homo sapiens, referred to as "Earth-descended" or "ED" humans, or as "Solarians", are the only sentient species aggressive enough to put up a good fight. (Human beings are called "Earth-descended" because billions of them live on Mars, Venus, and hundreds of other planets across the Galaxy.)

Allies of the Earth-descended humans include the telepathic "Carmpans", a subtle and mysterious species incapable of direct aggression. The first stories in the series are related by an individual Carmpan, the "3rd Historian", who seeks to chronicle life in the Galaxy and the struggle against the Berserkers.

The first story, "Without a Thought" (1963), was basically a puzzle story, in which the protagonist faces a problem of simulating intelligence to fool an enemy trying to determine whether there was any conscious being present on a spaceship.

Saberhagen came up with the Berserker as the rationale for the story on the spur of the moment, but the basic concept was so fruitful, with so many possible ramifications, that he used it as the basis of many stories. A common theme in the stories is of how the apparent weaknesses and inconsistencies of living beings are actually the strengths that bring about the killer machines' eventual defeat.