Sentinels were antimutant robots created by Bolivar Trask and developed by Trask Industries. According to Trask, they were named after the ancient guardians that stood at the gates of the citadel.

Sentinels are designed to hunt down mutants by specifically targeting the mutant X-Gene. By 2023, the Sentinels also targeted humans with the potential to have mutant offspring. Built out of a space-age polymer, the Sentinel Mark I model measured 18 feet tall and weighed approximately 4,800 lbs, being equipped with Dual Trask 873K energy blasters for armament support, and was able to achieve Mach 0.9 through the addition of Two G302-TI-99 enhanced performance ion thrusters. Each Sentinel was also fitted with a Mutant Detection Device, capable of distinguishing between mutant and human individuals and specifically targeting those with the mutant X-Gene.