Spectral energy distributions of blazars: Facts and speculations

We discuss the present knowledge about the Spectral Energy Distributions {SEDs) of blazars within a unified approach emphasizing overall similarities. The properties of the average SEDs of different samples of blazars suggest that more powerful sources contain on average less energetic particles. Detailed studies of TeV emitting blazars show that the energy of the particles emitting the bulk of the power increases during flares. A framework for a general theoretical understanding is proposed. We present recent results on the SEDs of a group of blazars with emission lines, allowing to estimate both the luminosity in the jet and the luminosity of the accretion disk. Implications for the origin of the power carried by relativistic jets are considered.