Just like evolving, experience in battles/contests/trials. I prefer not to have levels, as people say keeping track is to hard, and that they need to know how much experience they got from battle, and in the end it was a total shitstorm so I'm letting you guys have a bit of free reign with moves and levels, as long as it's not 'charmander I chose you! Alright now evolve twice and learn flamethrower on the first day!'