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