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Active Galactic Nuclei (AGN) shine over ~ 20 decades of the electromagnetic spectrum, from the radio to the gamma rays. In almost all of this huge energy range, AGN are the brightest sources in the sky, except for the relatively narrow (~3 decades) range from the infrared to the ultraviolet (UV).

The "modern history" of quasars is closely linked with the development of non-optical astronomy. In the early 60s, hard won precise positions of radio sources enabled the large optical telescopes of the day to take spectra of whatever optical object lay at that position. Some showed the starlight of normal galaxies, but often at large distances. In 1963 Marteen Schmidt made the "official" discovery of the first quasar, 3C 273, realizing that the point-like counterpart of a powerful radio source was at a redshift z = 0.16, implying an enormous luminosity. In the early sixties, the availability of new space-borne instruments also opened the new field of astronomical observations in the X-rays, which soon revealed a sky dominated by AGN.

It is now clear that the initial population of quasars discovered in radio surveys is only a small fraction of a 10 times more numerous class of quasars, most of which are "radio quiet". These sources are, in turn, a small fraction of the total AGN population, which is dominated by obscured sources that can only be detected through their hard X-ray emission or through their reprocessed radiation in the infrared.

In this chapter, we describe the main continuum properties of AGN across the whole electromagnetic spectrum, according to our present, still incomplete, knowledge. We do not treat the emission of the small minority of quasars known as "blazars", whose emission appears significantly altered by relativistic beaming effects. We focus on the observed properties of quasars from a phenomenological point of view. More emphasis on the physical processes responsible for the observed emission of quasars can be found in Armitage (this volume).

We start in Section 2 with unobscured (type I) AGN. We briefly discuss the main limitations in defining a representative sample of AGN. We then focus on a sample of bright, optically selected quasars (the Bright Quasar Survey, Schmidt & Green 1983) in order to build a mean Spectral Energy Distribution (SED) that takes into account all of the most recent observational results. This can be considered an update of the 10 year old compilation of Elvis et al. (1994; hereafter, E94). We conclude Section 2 with a brief overview of the possible dependence of the average spectral properties of type I AGN on redshift and/or luminosity and with a brief discussion on the often underestimated issue of the intrinsic dispersion of quasar spectra with respect to their average shapes.

Section 3 is devoted to obscured (type II) AGN. We first review the main spectral properties for each energy band. We then discuss the relation between dust and gas absorption, describing some recently discovered sources for which the ratio between gas and dust, and the dust properties, are strongly different from those in our Galaxy. Finally, in Section 4 we discuss the current methods that can be used to discover obscured AGN in the different energy bands. There are several indications that a significant fraction of these sources are still missing in current surveys. In particular, we focus on the techniques that can help to disentangle the AGN and stellar emission in the population of luminous sources that dominate the infrared and submillimeter sky. We conclude with a brief summary in Section 5.

We finish this introduction with a note on terminology: historically, the term "quasar" has been used widely to refer to AGN of high luminosity whose emission completely overwhelmed that of the host galaxy. (Thus, these sources appeared as point sources in optical observations.) Low-luminosity AGN were (and still are) referred to as "Seyfert Galaxies". However, evidence for any basic physical difference between these types of active objects has diminished through the years, essentially to the vanishing point. For this reason, in this chapter, we will use the term "quasar" as synonymous with type 1 AGN.

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