Thief deals damage and steals the target’s held item, if the user is not holding an item. Items stolen from wild Pokémon are kept after the battle ends, but items stolen from a Trainer’s Pokémon are not. Items stolen from the player’s Pokémon either by wild Pokémon or Trainer Pokémon are returned at the end of the battle. All held items in wi-fi/linked battles (including Battle Tower/Subway/Maison) are returned to their respective owners at the end of a battle. If the item would activate on attack (e.g. Focus Band), it does so before being stolen. If the item is consumable on attack (e.g. Focus Sash) then the item is consumed as normal and cannot be stolen. Pokemon with the ability Sticky Hold cannot have their held item removed. The Griseous Orb cannot be removed from Giratina; Type Plates cannot be removed from Arceus; Drives cannot be removed from Genesect; and Mega Stones cannot be removed from their respective Pokémon.
In Generations 2-5, Thief has base power 40 and 10 PP. In Generations 2-4, Thief steals a Trainer Pokémon’s item permanently, but a wild Pokémon or a Trainer’s Pokémon using Thief will not steal the player’s item at all.