Mega Evolution is a transformation affecting Pokémon. Mega Evolutions are identified by having "Mega" in front of their name.
In order to Mega Evolve, the Pokémon must be holding the appropriate Mega Stones (with the exception of Rayquaza which is only required to know the move Dragon Ascent).
Once per battle, only a single Pokémon for each Trainer may Mega Evolve. Switching the Mega Evolved Pokémon out will not dissolve a Mega Evolution. A Mega Evolved Pokémon can only revert back to its original state upon fainting or the conclusion of the battle. If a Mega Evolved Pokémon faints, but is later revived, that Pokémon cannot obtain a Mega Evolved state again for the remaining duration of the battle. Trainers on the same team during a Multi Battle will each be able to Mega Evolve once during the battle.
Mega Stones cannot be taken via Thief, Covet, Trick, or Switcheroo. Abilities such as Magician and Pickpocket will fail to snatch the Mega Stone from its holder and Pokémon with Klutz are able to use their respective Mega Stone. If used by the holder, moves such as Bestow, Fling, Trick and Switcheroo will always fail. Embargo and Magic Room also have no effect, and will not prevent a Pokémon from using its Mega Stone to Mega Evolve. Using Knock Off against a Pokémon holding its respective Mega Stone will not remove the item, nor will Knock Off's added effect of increased power on the turn it removes an item be granted. Likewise, Mega Stones also cannot be given to or taken by their respective species using any of the aforementioned moves or Abilities. 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.
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 with 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, Mega Sharpedo's Ability.
Since movement order is determined before any moves are used (except changes due to Quash and After You), the Pokémon's new Speed or Ability will not affect movement order during the turn which it Mega Evolves.
Delta Metagross (Ruin) can change into its Crystallized form when holding the Crystal Fragment. Despite the name difference, this form change functions identically to Mega Evolution, which means that Crystallizing Delta Metagross (Ruin) prevents other Pokémon in the party from Mega Evolving for the rest of the battle.
A Mega-Evolved Pokémon has different base stats, with the exception of HP, and can have a different Ability and type from its non Mega-Evolved form. Every Mega-Evolved Pokémon except for Mega Alakazam, Mega Eevee, and Mega Magcargo has a base stat total 100 points higher than that of its base form.