The Pokémon Let's Go games are the grand debut of the Pokémon series not only on the Switch, but on any home console - other than all the various spin-off games, of course. Anybody else remember Pokémon Snap on the N64?
The Let's Go! games hark back to one of the oldest Pokémon games, bringing the spirit of the Game Boy's Pokémon Yellow up to the modern day by having your choice of Pikachu or Eevee follow you around on the game map.
Your relationship with Pikachu or Eeevee (which depends on which version of the game you have) is different to that with any other Pokémon: not only do they sit on you while you travel through the game, but they automatically have a stronger bond, will learn special techniques, and can have their fur styled and be dressed up in various outfits.
There's one thing they can't do though: evolve.
Yep, that's right, just like in Pokémon Yellow, you can't evolve your chosen critter at all. Normally, Pikachu evolves into Raichu, and Eevee can be evolved into Flareon, Jolteon, or Vaporeon, depending on which elemental evolution stone you use.
If you try that in Pokémon Let's Go though, you won't have much luck. Use an appropriate evolution stone on your Pikachu or Eevee and the game will tell you that it doesn't like it, refusing to evolve - just like Pikachu did in the cartoons way back when. That means you're stuck with the cute, unevolved form whether you like it or not (though they're so adorable that how could you not?).
If you have any other Pokémon that you'd like to keep in their basic, unevolved form, you might want to check out our guide on how to stop any Pokémon from evolving.
Can you still get Raichu, Flareon, Jolteon, and Vaporeon?
Fear not, it's easy to get any of the evolved Pokémon anyway. Not only can you get them from trading, but you can also catch both Pikachu and Eevee in the wild, and these wild forms of the Pokémon can be evolved just as you normally would.
You'll just have to get hold of the appropriate evolution stone - either in the wild, or from the department store in Celadon City - and use it on the regular Eevee or Pikachu to force it to evolve just as you'd expect it to.