"GIrl Before a Mirror" represents the concept of vanity. It is a painting of a woman looking into a mirror, but the reflection staring out of the mirror is very different from the one looking in. There are many theories about the significance of this. The primary theory, however, is that the darkened face represents aging and death, thus the girl is confronting the inevitability of aging.
The muse for the painting was Marie Therese Walter, who was Picasso's lover when he painted "Girl Before a Mirror" in the 1930s, during his cubism period. Walter served as Picasso's muse for multiple works, explains The Museum of Modern Art. Some scholars believe there is an underlying sexual and even erotic undertone in the darkened, made-up face that is staring back at the fresh, more innocently-toned white and yellow faced girl. Others believe the difference in the reflections represents the transformation from young girl to woman. All theories regarding the meaning of the two reflections agree it is a representation, and perhaps a joining, of two selves. The painting also suggests that mirrors allow us to see within ourselves. In one side, the girl is clothed and in the other she is naked.