By level

This is the most common method: on reaching a set level, the Pokémon evolves after the battle. Charmander evolves at level 16 and Charmeleon at 36. To delay evolution, for example to learn a move earlier, just press cancel during the animation.

Stones and items

Evolution stones such as the Fire, Water, Thunder, Leaf and Moon Stone trigger evolution instantly at any level. Pikachu becomes Raichu with a Thunder Stone, and Eevee can become Flareon, Vaporeon or Jolteon depending on the stone.

Newer generations added more stones, like the Dawn, Dusk, Shiny and Ice Stone. Some Pokémon also evolve while holding an item when leveling up at a certain time, such as Happiny holding an Oval Stone during the day.

Trade

Kadabra, Machoke, Graveler and Haunter evolve when traded to another player. Others must be traded while holding an item: Onix becomes Steelix and Scyther becomes Scizor with a Metal Coat. Some recent games offer alternatives for solo players, so check the rules of the game you are playing.

Friendship, time and location

Pokémon like Pichu, Golbat and Eevee evolve when their friendship with the trainer is high. Walking together, winning battles and using care items raise friendship. For Eevee, time matters: with high friendship it becomes Espeon by day and Umbreon at night.

Some only evolve in specific places, like near a Moss or Ice Rock (Leafeon and Glaceon in the generations that use that method) or in areas with a magnetic field (Magneton and Nosepass). In recent games, several of these rules were replaced by stones.

Moves, gender and odd cases

Knowing a move can be the condition: Piloswine becomes Mamoswine when leveling up while knowing Ancient Power. Gender counts too: only male Kirlia becomes Gallade with a Dawn Stone.

There are some odd methods. Inkay evolves if the console is held upside down as it levels up, and Galarian Farfetch’d must land three critical hits in one battle to become Sirfetch’d. Each Pokémon page at the Tavern shows the method for every stage of its evolution line.