  Optical spectral variability of quasars and BL Lac Objects is compared by
means of the spectral variability parameter beta (Trevese & Vagnetti 2002).
Both kinds of objects change their spectral slopes alpha, becoming bluer when
brighter, but BL Lac Objects have smaller beta values and are clearly separated
from quasars in the alpha-beta plane. Models accounting for the origin of the
variability are discussed for both classes of objects.
