  Average Spectral Energy Distributions (SED) for different subgroups of
blazars are derived from available homogeneous (but small) data sets, including
the gamma-ray band. Comparing Flat Spectrum Radio Quasars (FSRQ) with BL Lacs
extracted from radio (RBL) or X-ray surveys (XBL) remarkable differences and
similarities are apparent: i) in all cases the overall SED from radio to
gamma-rays shows two peaks; ii) the first and second peak fall in different
frequency ranges for different objects, with a tendency for the most luminous
objects to peak at lower frequencies; iii) the ratio between the two peak
frequencies seems to be constant, while the luminosity ratio between the high
and low frequency component increases from XBL to RBL and FSRQ. The variability
properties, (amplitude and frequency dependence) are similar in different
objects if referred to their respective peak frequencies. Finally, comparing
spectral snapshots obtained at different epochs, the intensities of the two
components at frequencies close and above their respective peaks seem to be
correlated. The relevance of these properties for theoretical models is briefly
discussed.
