Sneak

Sneak is a Fallout, Fallout 2, Fallout 3, Fallout Tactics, Fallout: New Vegas, Van Buren and J.E. Sawyer's Fallout Role-Playing Game skill.

In Fallout and Fallout 2 you cannot run and sneak at the same time unless you have the Silent Running perk.

Fallout, Fallout 2 and Fallout Tactics

 * $$\text{Initial level}=25\%+(1\%\times\text{Agility})$$

Example: A starting Agility of 5.
 * $$25\%+(1\%\times5)=30\%$$

Sneak only covers sneaking as such, while pickpocketing is covered by the steal skill.

Fallout 3

 * $$\text{Initial level}=2+(2\times\text{Agility})+\left\lceil\frac{\text{Luck}}{2}\right\rceil$$

Example: A starting Agility of 5 and Luck of 5.
 * $$2+(2\times5)+\left\lceil\frac{5}{2}\right\rceil=15$$

Sneaking can be one of the most important skills in Fallout 3 for certain characters. Sneaking up on a target allows you to get closer, allows you to avoid taking damage for the longest time possible, and maximizes your damage potential through the use of sneak-attacks. Sneaking increases your ability to loot and steal, and allows you to avoid undesired confrontations, working as an offensive and defensive skill.

Unlike its predecessors, in Fallout 3 a player can both sneak and run at the same time regardless of perks, however, without the Silent Running perk the player will suffer a detection penalty while running.

Sneak states
The sneaking state is displayed while crouching, and alerts the player to detection and possible detection. Note: As soon as the conditions are satisfied for any state it replaces the one above it on the HUD even though the player may still be in the [HIDDEN] state to several targets around them. Essentially, it reports the worst detection state towards the Player of all potential targets. Once all targets that satisfy the most-elevated state are killed (as in you kill everyone actively pursuing you in [DANGER] ), the state will revert to the highest remaining state among the remaining targets.
 * [HIDDEN] means you're undetected. In this state, attacking a target guarantees the sneak attack critical bonus, pickpocketing is available, and it is safe to steal, lockpick, or otherwise access "owned" property.
 * [DETECTED] means you've been detected by one or more friendly targets. Pickpocketing is enabled, but almost always fails. Attempting to steal, lockpick, or otherwise access "owned" property will fail and result in friendlies turning hostile. Sometimes an unfriendly target will see you and this will be shown instead of caution because they haven't identified you as hostile or a target.
 * [CAUTION] means one or more unfriendly targets are aware of your presence and are searching for you. After a time the enemy will dismiss the disruption and the state will return to [HIDDEN] . Pickpocketing is disabled for the enemy searching for the player. Attacking in this state will NOT cause a sneak-critical hit.
 * [DANGER] means you've been detected by one or more unfriendly targets, which have determined your location. If [DANGER] is flashing, it means that the enemy can currently see you, if you break the line-of-sight enemy will still know your last spot and general location until the alert status goes down. The state can be returned to [HIDDEN] by moving away and staying out of sight for a long period of time.

Factors in successful sneaking

 * Sneak skill: The sneak skill applies whenever the player crouches. The higher the player's Sneak skill, the easier it is to remain undetected while attacking, stealing, or picking someone’s pocket.
 * Cloaking fields: The Stealth Boy and Chinese stealth armor will wrap the user in a cloaking field that makes him or her more difficult to detect while sneaking, rendering the player virtually invisible even in plain sight. The stealth field effect is extremely powerful, and its value matters very little, so the Chinese stealth armor's stealth field of 5 works almost (but not quite) as well as the Stealth Boy's 75. It should be noted that this field only increases one's sneaking ability, and the field will be negated if the player breaks sneak mode in any conventional manner, such as firing a loud gun. The stealth suit only has a +15 sneak versus the Stealth Boy's +100, but its effects are not limited by time. Cloaking can also be stacked, so the stealth armor and Stealth Boy can give a player a combined field of 80.
 * Enemy's Perception: Enemies with low Perception are easier to sneak around than enemies with high Perception. A target's Perception will be impaired (a -4 reduction) if they have a crippled head.
 * Line of sight: While out of sight an enemy can only detect the player through sound.
 * Light level: The player is more difficult to see in dark areas or at night if outdoors. The Pip-Boy light, while active, will make the player much more visible. Likewise, glowing weapons such as the Shishkebab, plasma rifle, shock baton and Jingwei's Shocksword can give away the player's position. Tesla armor is especially difficult to sneak in due to its high weight and many bright lights. Light effects can be partially countered by a stealth field, but never completely.
 * Movement: The player is hardest to detect when motionless and easiest to detect when running. You are able to avoid the sound aspect of this by getting the perk Silent Running. Moving faster while within line of sight will still make you easier to spot, but you will no longer get any penalty for movement speed to your sound made. NOTE: Moving the camera (and thus moving your body) counts as movement, but not as much as real movement. If you are in shadows with a direct line of sight to a target not aware of your location, moving the camera may very well cause the target to investigate, or immediately spot you.
 * Contact: Touching or running into targets whilst crouched will immediately alert them to your location. Regardless of how hidden you are, if you bump into a target, friendly or otherwise, you will immediately be detected. This also applies to robots that have been deactivated using the Robotics Expert perk. This does not apply if the enemy bumps into you; the enemy can push past you without becoming aware of your presence.
 * Equipped weight: The total weight of all equipped items (weapons count only if they are in your hands) makes your footstep noise louder for non-player characters. Silent Running removes this penalty entirely, allowing you full sneaking ability in any equipment. You will still hear your footsteps while sneaking with the perk, but non-player characters never will.
 * Sound: Most guns make noise. Louder guns make you easier to detect. The silenced 10mm pistol, dart gun, Infiltrator, Perforator, all melee weapons, and all unarmed weapons are considered entirely silent, and thus will not break sneak mode. Grenades will also not break sneak mode, as they do not make a noise at your location. Hitting but not killing an enemy with silent weapons still causes a CAUTION alert. All other weapons will most likely be heard, depending on your distance from the target. Missile launchers and Fat Man units make noise at your location and then at the impact point as well. Oddly enough, your Pip-Boy's radio has no effect on sneaking.

Sneak attacking
See Sneak attack critical.

Pickpocketing
While sneaking and HIDDEN, activating an non-player character will allow you to pickpocket them; in other words, steal items that they are personally "holding." This will open both your and your target's inventories. Items can be transferred either way, and thus items can also be planted on non-player characters. This is known as reverse-pickpocketing.

For each item you steal or plant in an non-player character's inventory, there is a chance (presumably based on your sneak skill, Luck, item weight/value and the non-player character's Perception) that you will be detected. If detected, the non-player character will automatically reclaim the item(s) you took and you will not be able to pickpocket that non-player character again (leaving the area and returning 24 hours later should allow you to re-attempt this).

You cannot steal items an non-player character currently has equipped. If you want to steal an item the non-player character is wearing or using (armor or weapon), you can reverse-pickpocket a better item into their inventory and then leave and re-enter the area. The non-player character will equip the item with the highest stats and you will be able to steal the item he or she was using before. For melee weapons, reverse-pickpocket a better ranged weapon onto the non-player character along with the appropriate ammunition. When the non-player character is reloaded, it will equip that weapon and unequip the melee weapon. Steal the melee weapon and the ammunition you gave them, and reload the non-player character. The non-player character will be unarmed and you can steal your ranged weapon back. Keep in mind that just because you do more damage with a specific ranged weapon doesn't mean that the non-player character will (usually but not always). Experiment to get the desired effect.

Successfully stealing the ammunition from an non-player character with a gun equipped will immediately render that non-player character unable to fire their gun. The amount of ammunition an non-player character is carrying will not match the amount they could fire if not pickpocketed (which is actually unlimited as long as they have 1 round for that weapon); this discrepancy can be attributed to gameplay purposes. This tactic can be used to influence the outcome of both non-player character/PC and non-player character/Non-player character conflicts. Concerning non-player character/Non-player character conflicts, this allows you to remain non-hostile to non-player characters on both sides of a conflict (by not needing to intervene with gunfire) while still ensuring victory for one side or another.

Non-player characters will notice you pickpocketing other non-player characters, hence the DETECTED sneak state, but in large open areas one can maneuver into gaps or around corners to avoid line-of-sight and return to HIDDEN. Non-player characters laying in beds may appear to have their back to you and still detect you, this is because as far as the game is concerned they are not facing the way they are laying down, that is merely an animation. To get "behind" an non-player character laying in bed, go to the foot of the bed.

The player can also use pickpocketing offensively to plant a live explosive. This will detonate after a short time, automatically killing the target regardless of the player's explosive skill or the target's health or resistances.

The Mothership Zeta add-on added another way to pickpocket a friendly non-player character. Reverse-pickpocket a cryo mine or cryo grenade onto the non-player character. After exiting the pickpocket menu, the cryo charge will go off but only stun or knockdown the non-player character. The non-player character can then be pickpocketed like any disabled target, with the exception that the non-player character won't turn hostile when he or she recovers.

Stealing Items
Picking up non-player character owned items in the game world while sneaking and HIDDEN, will allow you to steal items undetected.

Ways to increase Sneak

 * Bobblehead - Sneak (permanent +10 sneak)
 * Chinese Army: Special Ops Training Manual (permanent +1 or +2 with Comprehension)
 * Thief (permanent +5 with each rank)
 * Tag! (permanent +15)
 * Silent Running (permanent +10, removes movement speed penalties to sneak)
 * Fire ant nectar (temporary +2-8 depending on current Agility, a perfect Agility of 10 will see no benefit.)
 * Recon armor (temporary +5)
 * Shady hat (temporary +5)
 * Poplar's hood (temporary +10, additional +2 if less than 10 Agility)
 * Stealth Boy (temporary +100, +75 cloaking field)
 * Chinese stealth armor (temporary +15, also adds +5 cloaking field while crouched, requires Operation: Anchorage)
 * Lag-Bolt's shades (temporary +3, requires Broken Steel)

Sneak in quests and exploration

 * 25 Sneak can be used to convince Trudy to fight the Powder Gangers as an alternative to 25 Speech.
 * 50 Sneak is required to smuggle in hold out weapons at the New Vegas Casinos.
 * 50 Sneak when talking to Holdout about why you would need a weapon in casinos.
 * 75 sneak is needed to convince Father Elijah that hiding in the Sierra Madre is easy, and effectively get him to come down into the Sierra Madre Vault.

Ways to increase Sneak

 * Chinese Army: Special Ops Training Manual (permanent +3 or +4 with Comprehension)
 * Tag! (permanent +15)
 * Fire ant nectar (temporary +2-8 depending on current Agility, a perfect Agility of 10 will see no benefit.)
 * Recon armor (temporary +5)
 * Stealth Boy (temporary +100, +75 cloaking field)
 * Chinese stealth armor (temporary +5)
 * ¡La Fantoma! temporary +10 (+20 with Comprehension)
 * Assassin's suit (temporary +10)
 * Salt-Upon-Wounds' helmet (temporary +5)
 * Stealth suit Mk II temporary +15 (+25 With the Firmwire 1.1 Upgrade)
 * Dead Horses stalker armor (temporary+5)
 * Blood-Nap (temporary+10)