Beat Up inflicts damage on the target from each conscious Pokémon in the user's party that does not have a non-volatile status. Each strike is typeless and has a base power of 10; damage is determined using each attacking Pokémon's base Attack and level and the target's base Defense. Beat Up ignores changes to stat levels such as Swords Dance. Same-type attack bonus is not considered when calculating damage. Each strike is preceded with the message "<name>'s Attack!", where <name> is the name of the Pokémon in the player's party.
Each strike has an independent chance of being a critical hit, as well as activating a King's Rock. Beat Up will end instantly upon breaking a substitute. Bide and Counter deal damage based on the last strike alone. If Beat Up activates a target's Focus Band, no subsequent hits can knock it out.
Generations III and IV
Beat Up may continue attacking after breaking a substitute. If a Pokémon uses Beat Up on its partner in a Double Battle, the partner will attack itself (unless it suffers from a status ailment). If Beat Up activates a target's Focus Sash, no subsequent hits can knock it out. Beat Up ignores Wonder Guard, as it is considered a typeless attack. The Sniper Ability does not affect critical hits. Each strike has an independent chance of activating a Razor Fang.
Generation V onward
Instead of using each party member's Attack stat, the user's Attack alone is used to calculate the damage of each strike. Because of this, Beat Up is now affected by boosts like from Swords Dance and Attack-boosting items like Choice Band. Furthermore, the base power per strike is no longer 10, but instead individually based on the Attack base stats of the party Pokémon:
Base power = (party member's base Attack) / 10 + 5
Beat Up now receives STAB from Dark-type users and no longer deals typeless damage.
Beat Up no longer lists the user's name. Each strike will activate Justified and Weak Armor. If a Focus Sash, Focus Band, or Sturdy activates before the last strike is dealt, the following strike will cause the defending Pokémon to faint. A Focus Band can still activate repeatedly to prevent fainting, but each chance is independent. Each strike has an independent chance of activating Stench, but this does not stack with the Razor Fang or King's Rock.