Evolution

Evolution is the first evolution in the Pokémon franchise when one Pokémon, upon reaching a certain level, using a certain stone, learning a certain move, or being traded, evolves into a different kind of Pokémon. In Pokémon Gold, Silver, Crystal, HeartGold and SoulSilver games, it is stated that Professor Elm is an expert on evolution, and discovered that Pikachu evolves from Pichu.

In the anime, during evolution, a Pokémon will become surrounded by a light and slowly change shape. In the original, Advanced, and Diamond & Pearl series, the Pokémon is surrounded by a white light, while in the Best Wishes!, XY, and Sun & Moon series, the Pokémon is surrounded by a golden or blue light.

To some Pokémon, evolution means growing up, while to others, it just refers becoming another species or getting upgraded. However, many of them retain the memories of their life during their pre-evolution form.

Normal Evolution tricks
Pokémon gain experience after battling wild Pokémon and Pokémon Trainers. The more experience a Pokémon gains, the more it levels up. When a Pokémon reaches a particular level, it will evolve into its next form (if it has one). This is very helpful to most Trainers who want their Pokémon to become stronger.

There are ways to share experience, such as allow a Pokémon you wish it to evolve to hold Exp. Share. It is a hold item that allows the user to earn experience even if it did not participate in the battle. This is a convenient tool if you want to level up a new or low-level Pokémon.

You can prevent evolution by simply press the 'B' button on the Game when the Pokémon is attempting to evolve. This is useful as some Pokémon will learn moves that cannot be learnt in a later form or if you want a Pokémon to learn a move earlier than usual. For example, Gabite. If you let Gible evolve into Gabite at level 24 it will learn Dragon Rush at level 49, but if you keep Gible it will learn Dragon Rush at level 37. Sometimes it doesn't matter if you let it evolve or not. The Pokémon can hold an Everstone, which keeps them from evolving, so that you don't have to keep cancelling the evolution every time the Pokémon levels up. However, if your Pokémon has evolved, it might not evolve again (Raticate, Linoone, etc.) Although some Pokémon can evolve more than once (Poliwhirl, Cascoon, etc.), some basic Pokémon just can't evolve, due to undiscovered forms, or if they are really rare (Chatot, Groudon, etc.) There are also Pokémon that can evolve into different things like love, choice, etc. For example, Eevee, Poliwhirl, Wurmple, or Kirlia.

Stone Evolutions
Some Pokémon will not evolve unless you use a special stone called an Evolutionary Stone. These special Items are linked to the Pokémon's type.

Primal Reversion
Primal Reversion is a similar state to Mega Evolution, but the Pokémon devolve to a previous state in the past.

It is a possible game mechanic in Pokémon Omega Ruby & Alpha Sapphire, most likely because this mechanic was founded during development of Pokémon Omega Ruby & Alpha Sapphire.

Other
Devolution is a form of reverse evolution when a Pokemon reverts to a previous state. Devolution does not exist in the games but is common in the Manga and TCG. Devolution is achieved mainly unnaturally - through experimentation, or Devolution Spray. Though certain Pokemon, such as Isamu's Clefairy and Pikachu are capable of devolving themselves. The TCG also has it that Eeveelutions can revert into Eevee and that Mew has two special attacks named the Devolution Beam and Devo Crush - both of which which devolves the Pokemon hit by the attack. Golurk and Jirachi also possess special attacks which allow them to devolve an opposing Pokemon.

Anime
In the Anime Episode, Electric Shock Showdown, Misty points out that once Pikachu evolves with the use of the Thunder Stone, Pikachu won't revert back.

In An Epic Defense Force, Luke's Golett seemingly evolves to Golurk in a Movie then Devolves back to Golett, repairing the Golurk Statue, as it turns out to be fictitious.

Another instance of the non-existence of Devolution, is when Bonnie requested Ash's Frogadier to devolve back to Froakie, Clemont said that it would be impossible.

Trivia

 * There are eight Pokémon that can evolve into two different Pokémon, but only two that evolve into more: Tyrogue and Eevee.
 * As of Generation VII, there are sixty-six Pokémon that do not evolve in any way. This is discounting the fifty-six Legendary Pokémon and seventeen Mythical Pokémon.
 * In Dream a Little Dream from Me!, there is a group of Pokémon who evolved and devolved unnaturally, one actually missing its second evolved form.