  Although the gamma-ray emission in blazars dominates the power output, there
are crucial informations carried by the X-rays. Indeed, their paucity, together
with the short variability timescales observed both at X and gamma-ray energies
constrains the location, along the jet, of the active region responsible for
most of the emission. Furthermore, the knowledge of the X-ray flux and spectrum
helps in defining the overall spectral energy distribution (SED) of blazars,
allowing to discriminate between models.
