Pokémon X and Pokémon Y were always going to mark a big change in the long-running RPG franchise, simply because they represent the first games in the saga's main series to be realised in polygon 3D rather than flat top-down sprites.
Now we're starting to hear about the gameplay changes that will differentiate Pokémon X and Pokémon Y from what went before, and top of the list is the introduction of Mega Evolutions. This is an ability that certain Pokémon will have in battle, allowing them to temporarily transform into an incredibly powerful "mega" form - gaining new abilities and even changing type. The effects are limited to battle, however, and are not permanent. To mega evolve a Pokémon, you'll need to find the relevant Mega Stone for that creature.
It's unclear how many of the existing Pokémon will be able to enjoy this extra evolution, but Nintendo has confirmed that Mewtwo, Blaziken, Lucario, Ampharos, Absol and Mawile are among those that can.
Pokémon X and Pokémon Y are out on October 12th, exclusively for Nintendo 3DS. A special download event will accompany the launch, offering a mega stone equipped Torchic.