Mega Evolution is a transformation that certain Pokémon can undergo while in battle. This transformation changes the Pokémon's appearance and increases its stats, and may also change the Pokémon's type or Ability. Mega Evolution cannot be used outside of battle.
To wield Mega Evolution, a player must first obtain the following:
To obtain a Mega Bracelet, a Mega Evolved boss Pokémon must be found and defeated by the player. Upon doing so, the player will be given the option to wear the Mega Bracelet, which is necessary to trigger Mega Evolution.
Mega Stones can be obtained from their corresponding Mega Evolved boss Pokémon. The first defeated Mega Evolved boss Pokémon of each Pokémon species will always drop its Mega Stone, while subsequent boss Pokémon defeats will have a 1 chance of dropping the corresponding Mega Stone again.
For example, a player who has not obtained any Mega Stones or a Mega Bracelet defeats a boss Mega Venusaur. The Venusaur is guaranteed to yield a Mega Bracelet and a Venusaurite. If the player finds another boss Mega Venusaur, there is a 1 chance that the boss will drop a second Venusaurite for the player upon defeat. If the player later finds a boss Mega Blastoise, the Blastoise will be guaranteed to drop a Blastoisinite because it is the first Mega Blastoise that the player has defeated.
Once all three requirements for Mega Evolution have been obtained, the player can Mega Evolve a compatible Pokémon in battle by having the Pokémon hold its Mega Stone. The player must also be wearing the Mega Bracelet; Mega Evolution will be unavailable if the player takes off a Mega Bracelet.
Before the compatible Pokémon uses a move, there will be an option to Mega Evolve. If this option is selected, the Pokémon will Mega Evolve once the turn begins, before any moves are used. The stat, type, and Ability changes caused by Mega Evolution take effect immediately with the exception of Speed; turn order is decided before Mega Evolution, and any changes to the Mega Evolved Pokémon's Speed will only be taken into account from the next turn onwards.
Mega Evolution will persist throughout the battle, including when the Mega Evolved Pokémon switches out. The Pokémon will revert back to its base form if the battle ends or the Pokémon faints. Additionally, only one of a player's Pokémon may Mega Evolve during a battle; if the player has two Pokémon that can potentially Mega Evolve, Mega Evolving one of the Pokémon will cause the other Pokémon to be unable to Mega Evolve during the battle.
NPC Trainers do not require a Mega Bracelet to use Mega Evolution, but still require a Mega Stone to be held by a compatible Pokémon. NPC Trainers in higher-leveled Gyms may occasionally use Pokémon that can Mega Evolve.
Mega Evolution can be prohibited in a battle by enabling the Mega Clause battle rule. This will also prohibit the use of any Pokémon that are holding their Mega Stones.