active galactic nuclei are among the most fascinating objects in modern astrophysics. their radiation spans the electromagnetic spectrum from radio waves to gamma rays and exhibits large amplitude variability on many different time scales. research in active galactic nuclei utilizes almost all types of existing astronomical facilities. in recent years both ground-based telescopes and space-borne instruments have greatly contributed to our understanding of these objects. the conference on `active galactic nuclei across the electromagnetic spectrum extensively reviewed observational results and discussed theoretical models of active galactic nuclei. it offered a forum to present observations from many different wavebands and provided an opportunity to outline a synthetic picture of the active galactic nuclei phenomenon. the volume includes almost all of the 250 contributions presented during the conference. extensive reviews are presented as well as a large number of original results. the book covers a large domain of current research on active galactic nuclei. it will serve the existing research community in this field as well as provide a starting point to newcomers.