Morrowind:Sneak

The Sneak discipline is the art of moving unseen and unheard. Skilled sneaks are also adept pickpockets. Note that the skill does not improve simply from use and having the "sneak icon" on the screen. Despite the icon display, the skill only improves when within the detection range of a creature where they would notice the player. Sneak is a very useful skill, as it allows for critical hits.

Character Creation
The following races receive bonuses to their Sneak skill:
+10 bonus: Wood Elf, +5 bonus: Khajiit
The following classes have Sneak as a Major skill:
Acrobat, Agent, Assassin, Monk, Nightblade, Scout, Thief
The following classes have Sneak as a Minor skill:
Archer, Witchhunter



Factions
The following factions include Sneak as one of their favored skills:

Berne Clan
Blades
Camonna Tong (not joinable)
Census and Excise (not joinable)
Morag Tong
Thieves Guild
Twin Lamps

Sneak Mechanics
While Sneaking, a Sneak Check is performed periodically - once every couple of seconds for targets not in combat, to every few frames for targets that are actively looking for you. The chance that you can successfully remain hidden can be generalized as  .

Elusiveness
The Player's Elusiveness is equal to  
 

Distance to target can be determined via the GetDistance Console function (bring up the console, click on the target, and type  ).

Spot Chance
The target's chance to spot the player while they are Sneaking is equal to 
 

The Direction Multiplier is equal to 1.5 while the player is on either side or in front of the target, and equal to 0.5 while behind the target

Notes
Unlike looking in owned containers, 'just looking' while pickpocketing is considered a crime and will get you a bounty if caught, as well as expelled if performed on a fellow faction member.
Items may randomly not be visible in target inventories when pickpocketing while detected or with a low chance of success.
Without an optional patch from the Morrowind Code Patch, pickpocket chance is effectively capped at 56% as it is rolled twice with a cap of 75% both when taking an item and when closing the pickpocket window. Item weight is also not factored in without the patch.
Pickpocket chance is determined by the value of the item(s), your Sneak level, Luck, detection state (whether or not the hand-and-coinpurse icon is visible), and your target's Sneak level. Invisibility and Chameleon affect your detection state rather than pickpocketing directly, and despite governing Sneak, your Agility has no effect on your pickpocketing chances  . Your Sneak skill level has the largest direct effect on your pickpocket success chance; however, even at 100 Sneak, pickpocketing high-value items is nearly impossible. One easy way around this, assuming you can obtain a Fortify Skill effect for spellmaking, is to create a custom spell or enchant with one or more 1-second Fortify Sneak 100 on self effects, and cast it immediately before attempting a pickpocket. Do not include any other effects, an extremely high Sneak is all that is needed.
If you are playing the base game without Tribunal or Bloodmoon, you will not be able to access a Fortify Skill effect and therefore must make a much stronger Fortify Luck spell or enchant. If you also have a low (<35 or so) sneak skill and find it hard to remain undetected, you may also need a Chameleon or Invisibility effect to remain undetected, and wait for the hand-and-coinpurse icon to show up before attempting to pickpocket.
An exploit for auto-raising Sneak skill: Being undetected within observation range of an NPC will raise Sneak continuously. If playing on PC, the Morrowind Code Patch project allows the Sneak mode to be toggled. Otherwise bind Sneak to a convenient key and hold it down with a small and heavy object, such as a hammer. On the Xbox, you can simply double click the joystick down to enter an always-Sneak mode to get the same effect. Leave the game alone for a while and the Sneak skill will keep increasing, as long as one or more NPCs are within observation range and are not detecting you. If you want to be guaranteed to not be detected while you are away, you can stack constant-effect Chameleon to (ideally) 100. This trick will only work if NPCs or creatures are within visual range to detect you, such as in a room full of people, or a busy area with walking NPCs. If you have low Sneak from character creation, you will almost certainly need to train it first or use strong Chameleon effects for this to be viable. 
The 'drain and train' exploit also works for free skill increases from trainers, much faster, but requires your attention. Create a custom spell to drain sneak, 99 pts (or lower if you only want to train to a specific level) for 1 second, and use any Sneak trainer. This can be done with any trainer if you also fortify *their* sneak past their lowest skill they offer to train. This does not work with OpenMW.
A less cheaty method is to Sneak up on animals outside of town.  This can be dangerous since they will usually attack you if you fail.  Sneaking up on Scribs is safe.  Fortified Luck and/or Agility can make this technique very effective – it can increase your Sneak by 2 or 3 points if your Sneak skill is quite low. 
The Chameleon effect will help hide you while sneaking.  This is best done by stacking items, spells, and potions to get 100 points of Chameleon.
If you have a low Sneak ability and you want to raise it, you can search for a shop or another building with only one NPC. A prime example of this is Sellus Gravius in the Census and Excise Office. Find a position such that while you are in Sneak mode, sometimes the NPC will look over their shoulder if you Sneak behind them. Approach the NPC and wait for them to face you. The NPC will rotate to face you for a few seconds as you move around them. Rotate them away from your Sneak position. 

Bugs