Cover improves a unit's Defense which directly reduces the Aim of enemies shooting at them; conversely, units receive a +50% Critical Chance bonus when targeting an enemy which is out of cover. Thus these two factors combine to make taking cover the most frequent, if not most important, way in which soldiers are capable of lowering their overall risk of receiving any damage. Cover is perhaps the single most important thing to consider on Impossible difficulty where enemies' Aim values are much higher and almost any shot can result in instant death.
Most cover elements in the game are susceptible to damage which can degrade or destroy the cover thus reducing or eliminating the cover's Defense bonus. Missed shots from laser or plasma weapons may accidentally damage or remove cover whereas cover can be deliberately damaged through the use of explosives (or a MEC's Kinetic Strike Module in XCOM: Enemy Within). Note that the use of explosives can start fires which may destroy even more cover and temporarily obstruct unit movement. Certain cover elements are also liable to exploding when damaged; these elements include most vehicles, gas pumps, UFO Power Core, and certain barrels and will damage any unit or cover element adjacent to them when they explode.
There are two types of cover available, usually referred to as partial cover and full cover.
- Partial (or half) cover confers +20 Defense
- Full cover confers +40 Defense
- Soldiers can also Hunker Down which doubles their cover bonus.
- In XCOM 2, Hunker Down confers a flat 30 defense, and increases the soldier's dodge by 20, giving them a chance to take half damage from hits.
- The Sniper class Low Profile ability makes partial cover count as full.
An Alloy S.H.I.V.'s hardened shell allows it to be used for partial cover. In XCOM: Enemy Within, a MEC Trooper with the One For All ability can be used to for full cover. These units can provide a soldier with cover when it is direly needed, such as if an alien pod is unexpectedly discovered after a unit ends its turn exposed. They also allow the precise positioning of units in cover where no naturally-occurring cover is available, and enable the use of squad tactics such as Mobile Fortress during missions.
- Main article: Flanking
Flanking is a simple mechanic which allows a unit to ignore the cover bonus granted to an enemy from objects in the environment. A successful flank will display a yellow shield in place of the usual red shield. However, flanking does not ignore a unit's innate Defense stat or any defensive bonuses received from abilities such as a Smoke Grenade or equipment such as Ghost Armor.
Cover behaves essentially the same in XCOM 2, with the exception of how it interacts with Concealment. An enemy or civilian unit's movement may place a soldier within their detection radius, yet not break concealment, so long as that soldier is not flanked by that enemy or civilian unit.