You have to give it a lightning stone.
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Pikachu will evolve into Raichu when given a thunder stone. If you mean Ash's Pikachu, then it very unlikely he will evolve his Pikachu, as Pikachu is the Pokemon Franchise's mascot.
Raichu can't evolve anymore. If you're asking how to evolve Pikachu into Raichu, use a Thunderstone.
Raichu is the last form of pikachu and its preevolution is pichu.
Pikachu turns into Raichu.
Raichu(with THunderstone) and FROM PIchu.