Lastly, the aliens are grouped based on their primary versions then the variants of this primary version are expanded for example, Sectoids are the primary version, while the Paladins in Chimera Squad are a Sectoid variant. Human enemies, which are the dominant force in XCOM: Chimera Squad will not be mentioned in this list, only the aliens and alien hybrids will be included. With each installment in the series, the enemies change and often become more or less of a nuisance than they were in previous installments.

Starting with XCOM: Enemy Within and going to the newest release, XCOM: Chimera Squad, this article will provide a list of every alien variant in the XCOM series as well as cover the best tactics to use against them. The aliens in the XCOM series vary in difficulty and danger, ranging from cannon fodder to apocalyptic threats, with the Extraterrestrial Combat Unit, XCOM, at the center of the war. The XCOM series is a turn-based strategy game of humans versus an alien invasion.