The mechanics of how thief abilities actually work in-game is left somewhat vague in the rulebooks. This page provides a gameable system.

DMs should keep in mind that if they make thief abilities bad, or a pain to use, thieves will suck pretty bad in their campaign. Thieves are not good at combat and can't cast spells. Don't gimp their cool abilities; they won't be overpowered.

Whenever possible, the player should roll the thief ability check. The only time this is impossible is for Find Traps (FT). Even for Hide In Shadows (HIS) and Move Silently (MS), oftentimes the DM can put the dice in the player's hands by postponing the roll until the moment of consequence.

PHB 28 says only one attempt per thief is allowed. However, a thief can try again after they gain a level, just like with open locks (OL).^Ex

Finding a trap causes the thief to locate both the trigger and any directly related mechanisms (such as a nearby opening where a gas is released).

Success means the thief is no longer a valid combat target until the round after they reveal themselves.

Can even be done in melee as long as all enemies can attack other characters (i.e. there are not so many enemies that they can't all get attacks on the other characters due to the limit on how many creatures can attack a single creature).

On a subsequent round, the thief can break HIS to obtain a rear attack + BS attempt on a target within 1”. The thief could also prepare an action to BS anyone coming within 1”.

Success on HIS also gives +1 to surprise if the thief is 90’ or more away from any moving/noisy allies, but this does not stack with MS, since they can't be used in conjunction (HIS is stationary, MS involves movement).

This is done in conjunction with a close-to-striking-range action.

Success means the thief obtains a rear attack + BS attempt in a following round against a target within melee range that is otherwise occupied (e.g. looking elsewhere, fighting other characters in melee). The MS dice should not be rolled until the last possible moment, in order to keep the player in the dark.

Success means the thief is not a melee target on the round of the BS (and of course not on the round they move-to-engage, either).

Success on MS also gives +1 to surprise if the thief is 90’ or more away from less-silent allies, but this does not stack with HIS, since they can't be used in conjunction (HIS is stationary, MS involves movement).

Backstab (BS) grants +4 to hit (does not stack with +2 for rear attack), and a damage multiplier based on the thief's level. The faces shown on the dice are multiplied; damage bonuses are not multiplied.

A BS attempt can be obtained in one of four ways:

1. The thief has surprise. Backstabs are possible on all surprise segments.
2. With HIS or MS, as detailed above.
3. Without HIS or MS if the thief gets a rear attack by some other means.
4. Narratively, as adjudicated by the DM. In all cases, the DM should give the the victim the benefit of any doubt of whether a backstab is possible.¹⁾

BS while dual-wielding grants BS with both weapons. When initiative is tied and the thief gets multiple attacks due to speed factor, if the thief has a BS attempt, all attacks are BS attempts.

Multiple attacks in a single routine all qualify for BS. In the case of multiple routines, only the first routine qualifies for BS unless attacking from the rear.

In the case of multi-class characters, backstabs can be performed with any sword, dagger, or club, even if it is not a thief weapon.

Thieves can wear armor other than leather, but are subject to penalties as per UA 72 (or UA 22 pre-errata). Make sure to read all the footnotes to that table.

Magic armor (other than leather) reduces the penalties by 2% per plus of the armor.^Ex

Monks also obtain the listed bonuses for wearing no armor.