Evolution is the process in which a Pokémon changes into it's next evolutionary stage.
It's not unheard of for some Pokémon to not want to evolve, for varying reasons, such as not liking their evolved forms or wanting to be stronger without evolving.
- Most Pokémon that can evolve become able to do so upon to reaching a certain level.
- Upon reaching a certain level of friendship.
- Upon level up while knowing a certain move or a move of a certain type.
- Upon level up while holding a certain item.
- Upon level up in a certain location.
- Upon level up at a certain time of day.
- Upon level up in a certain weather condition.
- Upon level up with a certain Pokémon or Pokémon of a certain type in the party.
- Upon level up while the Nintendo 3DS is upside-down.
- Upon level up with high enough Beauty.
- Upon being traded, sometimes while holding a certain item or for a specific Pokémon.
- Upon using an item.
- Being fed a certain amount of species-specific candy.
- A combination of any of the above.
Some Pokémon, such as Pikachu, will evolve into one of two different forms of their evolved form depending on whether it's within Alola or not. If it's in Alola, Pikachu will evolve into an Alolan Raichu, anywhere else, it'll evolve into a Kantonian Raichu.
Holding an Everstone will prevent most forms of evolution.
Surprising an evolving Pokémon will stop the evolution.