Mystique is a mutant metamorph. She can psionically shift the atoms and molecules of her body and of whatever clothing she is wearing so as to change her and its appearance. As a result she can cause herself to look and sound like an exact duplicate of any human, humanoid, or semi-humanoid being of either sex, wearing virtually any kind of clothing. Her control is so exact that she can precisely duplicate another person's retina pattern in her own eyes, finger, palm and skin-pore patterns on her own hands and skin, and vocal cords to match voices to the point of corresponding voiceprints.
Mystique, however, is limited by her inability to diminish or increase her own mass at will. As a result, while she can make herself look exactly like a person who is physically bigger than herself, she will not weigh as much as the real person does. Although she can maintain the form of a person of her height, weight, and build indefinitely (as with her own appearance as Raven Darkholme), the longer that she maintains the form of a person physically bigger than herself, the greater the strain she feels. If she maintains that bigger form for too long a period, the length of which is directly proportional to the difference in size between that form and her normal one, she will collapse and automatically revert to her normal form.
Mystique cannot duplicate the powers of the person she imitates: for example, when she turned herself into a duplicate of Nightcrawler, she did not gain the ability to teleport.
As yet Mystique has not demonstrated any ability to adopt the form of non-humanoid beings, animals, plants, or non-living objects. It is conjectured that she could not imitate any of these without interfering with the proper functioning of her internal organs and possibly dying, but the truth, like much regarding Mystique, remains shrouded.