Simultaneous multifrequency observations of blazars

Among active galactic nuclei, blazars have the most highly and rapidly variable spectral continua at all the wavelengths. Their repeated observation at different energies allows therefore to investigate the geometry and the physics of the emitting regions, provided the multifrequency data are monitored simultaneously and with complete and frequent sampling, to allow a reliable correlation analysis of the light curves at different wavelengths. The coordinated efforts of several international astronomical groups toward multifrequency monitoring implementation allowed to obtain unprecedentedly high quality data and to achieve substantial improvement in the theoretical understanding of the blazars PKS 2155–304 and 3C 279