XCOM is not just facing alien invaders or their ADVENT puppets. Some enemies are partially or fully mechanical. Such units are immune to the effects of fire or Poison and impervious some but not all psionic abilities. These units tend to be durable, with significant armor. However they do have several vulnerabilities not shared by their organic brethren. Robotic enemies may be hacked by Specialists, and are especially vulnerable to the Specialist's Combat Protocol and Capacitor Discharge. Other classes may use Bluescreen Rounds and EMP Grenades/Bombs which not only bypass the robotic unit's armor pips but have additional effects beyond mere damage.