Multiwavelength properties of active galactic nuclei

The emission of active galactic nuclei spans the electromagnetic spectrum from the far-infrared domain (or radio domain) to the gamma rays. This emission is made of a number of components, each described by a number of parameters which, at present, cannot be deduced from a comprehensive and self-consistent underlying model. The observations allow, however, to identify the physical origin of most of the components. Correlations between the light curves observed in different spectral domains have been found for few objects which have been intensively observed in the last few years. The observed lags have provided unexpected and important constraints for the interpretation of the emission components.