Cripple

Fallout 1 & Fallout 2
To cripple an enemy, aim for the arms and legs. If you have a crippled leg you can't run and you use more action points. If you have a crippled arm you can't wield two handed weapons. If the arm crippled is the arm wielding a gun, the accuracy of the weapon will decrease significantly. If both arms are crippled, you can only use unarmed attacks.

In Fallout 2, crippling of the eyes resulted in blindness.

Fallout 3 and Fallout: New Vegas
Although body part damage can be achieved through manual aiming, V.A.T.S. allows more effective targeting of particular body parts. The effects of crippling specific body parts are listed below.
 * Crippling the head will reduce overall accuracy and perception (-4) of the target. If inflicted upon the player, the screen will intermittently get fuzzy and a ring will be heard through the speakers, loosely simulating the effects of a concussion.
 * Crippling the torso will result in the target flinching far more often from shots than normal. This can also affect the player, throwing off aim and stopping your attacks momentarily.
 * Crippling an arm will drastically reduce accuracy and may disarm the enemy if a weapon is held in the attached hand. This is particularly evident when the player uses a scoped weapon such as a Sniper Rifle. The weapon itself is not destroyed, and the enemy can pick it up again or use another weapon from his or her inventory.
 * A crippled right arm also reduces base melee damage done by 10% if wielding a one-handed weapon. If wielding a two-handed weapon (or unarmed), one crippled arm reduces base damage done by 5%, two crippled arms reduce base damage done by 10%.
 * Crippling a leg will result in impaired mobility. Creatures who rely on leaping attacks (e.g. deathclaws) will no longer be able to use them.
 * Crippling the held weapon will ruin it and make it unusable, and/or send it flying out of the target's hand. Although the condition is zero and it can't be used, it can still be collected and repaired, or used for repairs. The player's weapon may be damaged by enemy fire as well. If the player's weapon CND reaches 0, it will be unequipped as opposed to flying out of the player's hands. If it is an explosive ordnance (like a frag grenade), it will explode, dealing its damage to the target holding it (and any others nearby). Grenades can be targeted by V.A.T.S. while in the air after being thrown as well.
 * Some enemies have alternate targets: Robots, ants, and others have special targetable parts that when crippled will push them into a frenzied state, where they no longer discern between friend and foe and will attack anything.
 * V.A.T.S can be used to target grenades or any other thrown explosive, whether in the enemy's hand or in the air. It will detonate if shot.

Crippling any body part will result in the target momentarily pausing to clutch the now crippled body part or reel from the injury.

When using explosive and melee weapons you cannot target specific body parts while in V.A.T.S. You simply target the enemy as a whole.

Healing a crippled body part
In combat, the player may also sustain crippling damage. This was originally to be a mini-game. In order to repair this damage, you must:
 * Pay (50-100 caps) for a doctor to heal you.
 * Sleep in any bed.
 * Use a stimpak (either on the crippled body part directly or generally through normal stimpak-healing).
 * Develop Advanced Radiation Sickness (400 Rads) if you have the Rad Regeneration Perk.
 * Use My First Infirmary.
 * Use a doctor's bag or hydra.

Hardcore Mode (Fallout: New Vegas only)

 * Paying a doctor works the same.
 * Sleeping only heals if the bed is owned.
 * Stimpaks can't be used to heal limbs.
 * Doctor's bags function like stimpaks on Casual.