In order to Mega Evolve, most Pokémon must be holding their appropriate
Mega Stone and the Trainer must have a
Key Stone. Only
Rayquaza does not need a Mega Stone to Mega Evolve; instead, it must know the move
Dragon Ascent.
In any battle, each Trainer may Mega Evolve a single Pokémon once. Mega Evolution is essentially a free action a Pokémon may make in addition to its normal move, but if a Pokémon will not use a move, it cannot Mega Evolve. Mega Evolution occurs
before most actions except
switching. A Mega-Evolved Pokémon will only revert back to its original state if it
faints or the battle ends; if it is switched out, it will still be in its Mega Evolved state if it is switched back in.
A Pokémon cannot Mega Evolve if it is being held in the airborne phase of
Sky Drop. Neither volatile nor non-volatile
status conditions such as paralysis, sleep, and infatuation will have any effect over a Pokémon's ability to Mega Evolve; even if they result in the Pokémon not being able to move that turn, the Mega Evolution will occur regardless.
Mega Stones are not affected by most moves or Abilities. If they would move a Mega Stone to or from a Pokémon that could use it to Mega Evolve, the moves
Bestow,
Covet,
Fling,
Thief,
Trick, and
Switcheroo will fail and the Abilities
Magician,
Pickpocket and
Symbiosis will not activate. The move
Knock Off cannot remove a Mega Stone from a Pokémon that can use it to Mega Evolve, and Knock Off's power will not be increased since it cannot remove the item. The moves
Embargo and
Magic Room and the Ability
Klutz will not prevent a Pokémon from using its respective Mega Stone to Mega Evolve.
Performing Mega Evolution will not alter the turn order, so a Pokémon with a lower Speed than their opponent will still go after them on the one turn the Mega Evolution occurs, even if their Mega Evolution increases Speed and would otherwise cause them to go first. The opposite is also true for Pokémon who lose Speed from Mega Evolution, such as
Garchomp. However, turn order will be determined normally on every turn afterwards. All other stat changes and type changes of a Mega Evolution will take effect immediately and damage calculation will proceed as usual. (A Pokémon whose Mega Evolution possesses a lower Defense stat, for example, will take greater damage from Physical attacks on the turn of Mega Evolving.)
Changes in a Pokémon's
Ability will also take place immediately. A
Sharpedo that normally has
Speed Boost will not gain a boost in Speed at the end of the turn it Mega Evolves, but if it uses a biting move such as
Crunch that turn, it will be boosted by
Strong Jaw, its Ability as Mega Sharpedo.
In
Pokémon Omega Ruby and Alpha Sapphire, Mega Evolution can also occur during the
Talent round of
Pokémon Contest Spectaculars. If the participating Pokémon is holding its corresponding Mega Stone when its
appeal causes the audience's excitement to reach its peak, it will Mega Evolve and perform a
Spectacular Talent. The Pokémon will remain in its Mega Evolved state for the remainder of the Talent round.