Astral Shift

Astral Shift (Japanese: メガシンカ Mega Evolution) is a temporary transformation affecting that was introduced in Generation I. Mega-Evolved Pokémon are identified by having "Mega" in front of their name.

There are 46 species of Pokémon that are capable of Mega Evolution, and there are 48 different Mega Evolutions.

History
Korrina, the Gym Leader of Shalour City, is said to know secrets about Mega Evolution. The Tower of Mastery is also related to Mega Evolution. According to a Kalos legend, the first Pokémon to have been Mega Evolved was a. However, according to Zinnia's grandmother in Pokémon Omega Ruby and Alpha Sapphire, was the first Pokémon to Mega Evolve; Mega Evolution was first discovered by the first meeting of humanity and Rayquaza. Professor Sycamore theorizes that Mega Evolution can only be achieved if there is a strong bond between a Trainer and the Pokémon.

In the games
In order to Mega Evolve, most Pokémon must be holding their appropriate Mega Stone and the Trainer must have a Key Stone. Only does not need a Mega Stone to Mega Evolve; instead, it must know the move.

In battle
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 to its original state if it faints or the battle ends; if it is switched out, it will remain Mega Evolved.

A Pokémon cannot Mega Evolve if it is being held in the airborne phase of. 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.

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, , , , , and will fail and the Abilities ,  and  will not activate. The move 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 and  and the Ability  will not prevent a Pokémon from using its respective Mega Stone to Mega Evolve.

Changes in a Pokémon's Ability will take place immediately. A that normally has  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  that turn, it will be boosted by, its Ability as Mega Sharpedo.

Generation VI
In Generation VI, performing Mega Evolution will not alter the turn order. 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. However, turn order will be determined normally on every turn afterwards.

Otherwise, stat changes and type changes due to Mega Evolution take effect immediately and damage calculation proceeds as usual. For example, a Pokémon whose Mega Evolution has a lower base Defense stat takes greater damage from physical moves on the turn it Mega Evolves. Likewise, the move considers the Pokémon's Speed stat after Mega Evolution for damage calculation, not before.

Likewise, even if a Pokémon only has an Ability such as before Mega Evolution, appropriate moves will gain priority. If a Pokémon gains Prankster upon Mega Evolution, appropriate moves will not gain priority.

When the player Mega Evolves a Pokémon for the first time since the save file is accessed, a longer animation than usual will play, showing the player activating the Mega Evolution with his or her Mega Bracelet. On subsequent uses of Mega Evolution, a shorter animation will play, showing only the Pokémon as it Mega Evolves.

Generation VII
In Generation VII, a Pokémon's Speed after Mega Evolution is used to determine turn order, not its Speed before. Likewise, if a Pokémon only has an Ability such as before Mega Evolution, appropriate moves will not gain priority. If a Pokémon gains Prankster upon Mega Evolution, appropriate moves will gain priority.

cannot Mega Evolve if it holds a Z-Crystal.

In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, as the Held item mechanic is not implemented, the player simply needs to have the compatible Mega Stone in their bag to access a Pokémon's Mega Evolution. The player can also manually pick which form Charizard or Mewtwo can become from the move menu.

Outside of battle
In Pokémon Omega Ruby and Alpha Sapphire, Mega Evolution can also occur during the 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. The Pokémon will remain Mega Evolved for the remainder of the Talent Round.

In Pokémon Omega Ruby and Alpha Sapphire, when summoned by the Eon Flute, or  will Mega Evolve before the player mounts it to soar in the sky. It will Mega Evolve even if it is not holding its corresponding Mega Stone, since the Eon Pokémon is summoned even if it is no longer in the player's game.

Effects
A Mega-Evolved Pokémon has different base stats, with the exception of, and can have a different Ability and type from its non-Mega-Evolved form. Every Mega-Evolved Pokémon (except for Mega Alakazam in Generation VI) has a base stat total 100 points higher than that of its base form.

Trivia

 * There is at least one Mega-Evolved Pokémon of every type: Psychic has the most (with ten), while Ice and Electric have the fewest (with two each).
 * Electric is the only type for which no new Mega Evolution was introduced in Pokémon Omega Ruby and Alpha Sapphire.
 * and are the only Pokémon capable of Mega Evolution that are in the same evolutionary family.
 * Only Pokémon which cannot evolve further are capable of Mega Evolution.
 * is the only Fossil Pokémon capable of Mega Evolution.
 * is the only trio master capable of Mega Evolution.
 * is the only Mythical Pokémon capable of Mega Evolution.
 * With, Mega Mewtwo X, Mega Mewtwo Y, and Mega Rayquaza have the highest base stat totals of any obtainable Pokémon. Eternamax has the highest base stat total of all Pokémon at 1125, but cannot be used by the player.
 * In addition, Mega Mewtwo Y has the highest base Special Attack of all Pokémon at 194, and Mega Mewtwo X has the highest base Attack of all Pokémon at 190. Mega Mewtwo X also has the highest base stat increase of all Mega Evolutions, its Attack stat being boosted by 80 upon the process.
 * is the only dual-type Pokémon to become single-type upon Mega Evolving, going from -type to pure.
 * The only Pokémon with multiple Mega Evolutions are and, which have two each. In both cases, the first ones to be revealed were the Y versions.
 * In addition, both of their Y Mega Evolutions kept their original type(s), while both of their X Mega Evolutions changed or added their secondary type.
 * Also, in both cases, the X Mega Evolution is more physically-oriented (with a higher Attack and/or Defense), while the Y Mega Evolution is more specially-oriented (with a higher Special Attack and/or Special Defense).
 * Rayquaza is the only Pokémon that does not require a Mega Stone to Mega Evolve in the core series games, instead being required to know.
 * This makes Rayquaza the only Pokémon that can have a different held item while Mega Evolved, since Mega Stones cannot be taken off their proper holder during battle.
 * In Pokémon Shuffle Rayquaza needs the player to get the Meteorite for it to Mega Evolve.
 * Mega and Mega  are programmed into ; as a result, they are not banned by the Kalos rules from battles, and their Mega Stones are not banned from trades between Omega Ruby or Alpha Sapphire and X or Y, despite being otherwise unavailable in those games.
 * The Japanese and Korean logos for Pokémon X and Y include the Mega Evolution sigil.
 * In an interview with the Nintendo Dream magazine, Ken Sugimori stated that was intended to get a Mega Evolution in Pokémon X and Y, but the idea was dropped  due to him experiencing artist's block.
 * In the Pokémon X and Y demo, the message "An immense rare power has risen in Mewtwo" would be displayed before Mega Evolving. In the final game, the message displayed is "'s  is reacting to 's !" Additionally, Mega Mewtwo Y is referred to as simply Mega Mewtwo.
 * is the only Pokemon to have both a Mega Evolved form and a regional form.