Ash's Pikachu, Squirtle and Bulbasaur have never evolved and they never will.
-Pikachu is a main character and therefore his appearance changing would be a ton of extra work, also it would not look right for the main character to change.
-Bulbasaur and Squirtle are no longer with Ash and Bulbasaur currently resides at Professor Oak's lab while Squirtle stays with the Officer Jenny in charge of the Squirtle Squad.
To evolve a Pikachu in-game, use a Thuderstone on it.
To evolve a Bulbasaur, level it up to level 16.
To evolve a Squirtle, level it up to level 16.
As seen in "Bulbasaur's Mysterious Garden" Bulbasaur tried hard not to evolve.
In the episode "Electric Shock Showdown" Pikachu had a chance to evolve into Raichu, but did not want to.
Ash's Squirtle has shown no opinion towards evolving further but because of its ties to the Squirtle Squad and its friends there, it's unlikely it will evolve.
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episode 1337
Bulbasaur does not evolve during the Pokemon anime series, there is an episode where Bulbasaur is about to evolve, but he restrains himself from doing so, because it didn't wish to evolve
Ash's Pikachu will not evolve because Pikachu chose to stay with Ash and gain more experience and in the episode that Ash had battled Lt. Surge Nurse Joy gave Ash a Thunderstone but pikachu don't want to evolve and Brock said that pikachu has more speed than Raichu
It Evolved prior to the episode Staging a hero's Welcome!, When it was traveling with May in Jonto!
Bulbasaur does not evolve with a stone. Bulbasaur needs to get to a certain level, then go to "Luminous spring", select Bulbasaur, then select "evolve". If YOU are Bulbasaur, then you must get Manaphy and Palkia to join your team, and then go to "Luminous spring".