Dr. Bolivar Trask was scared the the mutants would take over the human race some day. That is why he created the Sentinels, a group of robots that would hunt down mutants. Ironically, Trask also had two children who were also mutants: Tanya who had the ability of time travel and Larry who was a precog and also had a special amulet that suppressed his powers. First, Trask created a prototype called Master Mold who then create Mark I Sentinels. However, the Sentinels' logic decided that if they controlled the humans, they would take care of the mutants. It captured Trask and he realised that he was the one who had created the threat. To destroy Master Mold, he sacrificed himself by triggering an explosion. Larry was angered by this, blaming the X-Men and then built the Mark II Sentinels. These were smarter, as they could determine what kind of powers the mutants had and choose the best plan to capture them. However, Larry removed his suppressing amulet and the Sentinels then saw him as a mutant. He was killed in battle in which the Mark II Sentinels were destroyed.

Even though two versions of whole Sentinels have been destroyed, the technology still lived on as the US government took possession of Trask's plans amd hired Sebastian Shaw to build a new line of Sentinels.

For a brief time, Sentinels were reprogrammed by Onslaught to occupy New York and apprehend anyone they could find with superhuman abilities.

Nimrod

This Sentinel was created in an alternate reality "Days of Future Past" and was the most advanced Sentinel yet. He pursued Rachel Summers through to the X-Men's present and became a vigilante. He then evolved into a human-like Bastion.

Human Sentinels

Sentinels looked at humans as weak and Bastion converted humans into cyborgs called the Prime Sentinels.

Prime Sentinels

Bastion had outfitted some Prime Sentinels that resembled the original ones. With technological implants, they could fly and blast fire energy from their hands. Like all Sentinels, they were to erase the human race.