Originally posted by demigawd
There's a lot of debate about that, since she's been portrayed two different ways, alternately.1) She needs to breathe, and can only stay phased for as long as she can hold her breath
2)Her phased state is her natural state, and she has to concentrate to stay solid.
She's been written both ways, and there's never been consensus on which should be the way she really is. Personally, I prefer the former, but Marvel is pretty confused on which she should be. Are there any recent Directory entries of her? That will clear it up.
from wikipedia
Shadowcat possesses the ability to "phase" or literally pass through solid matter by passing her atoms through the spaces between the atoms of the object through which she is moving. Utilizing her phasing ability, she can also walk on air. While phasing, she is intangible and thus invulnerable to physical attacks and telepaths have a hard time reaching her mind in phased form. Her phasing powers also disrupt any electronic equipment she passes through. For some time after she discovered her powers, she was only able to phase herself and her clothes along with her. However, she quickly learned to phase other people and objects with her.
Due to a battle injury, Kitty Pryde became unable to control her phasing abilities and nearly faded out of existence. Although Reed Richards and Doctor Doom saved her life, for years she was only able to become solid through great effort; she has since overcome this disability.