When you reach 3rd level, you augment your martial prowess with the ability to cast spells. See Spells Rules for the general rules of spellcasting and the Spells Listing for the Arcane Trickster spell list.

Cantrips

You learn three cantrips: mage hand and two other cantrips of your choice from the Arcane Trickster spell list. You learn another Arcane Trickster cantrip of your choice at 10th level.

Spell Slots

The Arcane Trickster Spellcasting table shows how many spell slots you have to cast your Arcane Trickster spells of 1st level and higher. To cast one of these spells, you must expend a slot of the spell’s level or higher. You regain all expended spell slots when you finish a long rest.

For example, if you know the 1st-level spell charm person and have a 1st-level and a 2nd-level spell slot available, you can cast charm person using either slot.

Spells Known of 1st Level or Higher

You can prepare three 1st-level Arcane Trickster spells of your choice.

The Spells Prepared column of the Arcane Trickster Spellcasting table shows how many spells of 1st level or higher you can prepare at once.

You can also change your list of prepared spells when you finish a long rest. 

Spellcasting Ability

Dexterity is your spellcasting ability for your Arcane Trickster spells, since you cast your spells using quick and complicated hand movements. You use your Dexterity whenever a spell refers to your spellcasting ability. In addition, you use your Dexterity modifier when setting the saving throw DC for an Arcane Trickster spell you cast and when making an attack roll with one. 

Starting at 3rd level, when you cast mage hand, you can make the spectral hand invisible, and you can perform the following additional tasks with it:

You can stow one object the hand is holding in a container worn or carried by another creature.

You can retrieve an object in a container worn or carried by another creature.

You can use thieves’ tools to pick locks and disarm traps at range.

You can perform one of these tasks without being noticed by a creature if you succeed on a Dexterity (Sleight of Hand) check contested by the creature’s Wisdom (Perception) check.

Starting at 3rd level, you can use your Cunning Action to cast any of your Arcane Trickster spells.

Starting at 6th level you can replace a number of your prepared spells with new spells from the Arcane Trickster spell list equal to your proficiency bonus when you complete a short rest. 

Starting at 9th level, if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn. 

At 13th level, you gain the ability to distract targets with your mage hand. As a bonus action on your turn, you can designate a creature within 5 feet of the spectral hand created by the spell. Doing so gives you advantage on attack rolls against that creature until the end of the turn. 

At 17th level, you gain the ability to magically steal the knowledge of how to cast a spell from another spellcaster.

Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier against your spell save DC. On a failed save, you negate the spell’s effect against you, and you steal the knowledge of the spell if it is at least 1st level and of a level you can cast (it doesn’t need to be a Arcane Trickster spell). For the next 8 hours, you know the spell and can cast it using your spell slots. The creature can’t cast that spell until the 8 hours have passed.

Once you use this feature, you can’t use it again until you finish a long rest.