Sneak

Quiet movement, and the ability to remain unnoticed. If successful, you will be much harder to locate. You cannot run and sneak at the same time.

Certain weapons are more suitable to use while sneaking. The skill provides bonuses with such weapons, and if you combine it with certain perks, you might be able to perform devastating criticals!

You can only sneak while having a charged stealthboy in your inventory. When activating the stealth boy, you enter sneak mode, and the stealthboy consumes small energy cells over time. It can be recharged by using the engineering skill on it, or by putting it in your active slot and selecting the "recharge" option. You can see the remaining charges when the stealthboy is in your active slot, or by using the science skill on the stealthboy.

You can't enter sneak mode while being watched by somebody from another faction.

While sneaking, your perception is lowered by 3.

The Detection Formula

Detection Range = (Sight Range)*(1 - 0.2*{(Sneak - 50)/50 + Angle})

Where Angle is given by:
front: 0
front sides: 1
back sides: 2
back: 3

Sight Range is given by Perception*4 + 31, and is further modified by certain perks and traits. For example the "scoped" weapon trait gives +8 sight range per rank.

Wearing certain armours will modify your sneak skill for the above calculation: leather jacket, combat leather jacket and leather coat gives +25 skill, and everything else that isn't leather armor, leather armor mk 2 or fire leather armor (including bluesuit) gives -25 skill.

With the item perks Ghost and Ghost II (unlocked with skill 150 and 175), you will get a +20 skill bonus if you are within 1 and 5 hexes from a wall, respectively. Furthermore, if the path between the sneaker and the observer is obscured by a scenery that blocks movement but not sight, the sneaker gets +10 to his skill.

The minimum detection range (the range at which you will always detect somebody, regardless of his skill) is given by:

Minimum detection range = 0.16 * (100 - angle * 20)

Where angle is again defined as above.

Desneaking

Certain actions will desneak you: Running out of charges for the stealthboy, pressing the hotkey "1" to exit sneak mode, being shot, and attacking somebody with a weapon that doesn't have the weapon trait Silent Attack.

After you have been desneaked, you will be particularly vulnerable for 5 seconds. Any damage you take during that time will bypass parts of your armor, causing tremendous damage.

Attacking somebody with a weapon that has the trait "silent attack" will not desneak you. Instead, it will only "reveal" you for a short amount of time. How long depends on the weapon itself and can be seen in the weapon description. While revealed, you count as if you weren't sneaking for the purposes of detection. Assuming you don't get desneaked in some other way, after the reveal time has passed, you will return to being properly sneaked.