In the fictional Marvel Universe, Sentinels are technologically advanced robots, often gigantic in stature, that are designed to hunt down mutants, especially the X-Men. They were created by Stan Lee and Jack Kirby in X-Men volume 1 #14. They have evolved and improved in their capabilities with every new iteration.

Sentinels have a wide array of abilities; they can fly, shoot various weapons from different parts of their bodies, and detect mutants at long range; some of them can even change form and re-assemble after being destroyed. Most dangerous of all, some variants have the ability to learn from the attacks they face and create defenses against them which hampers the opponents' chances of defeating them. The "X-Sentinels" created by Steven Lang were capable of mimicing the appearances and powers of specific members of the original team of X-Men. Several groups of Sentinels have been created and/or led by a massive Sentinel named Master Mold.

The Sentinels were first created by Dr. Bolivar Trask, who intended to use them to save humanity from what he saw as a threat to the species' existence in the form of mutants. In a televised debate between Trask and Professor Charles Xavier, Trask revealed and then activated the Sentinels, who promptly decided that the best way to protect humanity was to rule over it themselves. The Sentinels kidnapped Xavier and brought him and Trask to the primary Sentinel, Master Mold, only for Xavier's students, the X-Men, to find them. When Trask realized the error of his ways, he sabotaged the machinery in the Sentinel base, destroying Master Mold and the Sentinels in an explosion, but he died in the process.

However, numerous Sentinels and several Master Molds have since been built since. Trask's son Larry built the next batch of Sentinels, only to be slain by them when he removed the medallion that his father had given him years ealier: the medallion blocked the Sentinels' mutant-sensing equipment, and Larry Trask was himself a mutant. The most long-lived Sentinel project was that of "Project Wideawake", a government agency led by Henry Peter Gyrich and Valerie Cooper that purchased Sentinels from Sebastian Shaw, the mutant Black King of the Hellfire Club.

In the "Days of Future Past" future timeline first seen in Uncanny X-Men #141, the Sentinels have become the de facto rulers of the United States. The Sentinels of this timeline are highly advanced in comparison to those of the present day, and the most powerful of these was Nimrod, who travelled back in time to the present in pursuit of Rachel Summers.

Latest appearances of Sentinels were in New X-Men #115 where Cassandra Nova uses Trask's grandson to revive a Master Mold in Amazonia which is sent to destroy Genosha and in Mekanix where a boat full of Sentinels come to Chicago where Kitty Pryde is attending University.

Now there seems to be no more Sentinels unless some writer decides to create some more.