  The spectra of Active Galactic Nuclei (AGNs) are often characterized by a
wealth of emission lines with different profiles and intensity ratios that led
to a complicated classification. Their electro-magnetic radiation spans more
than 10 orders of magnitude in frequency. In spite of the differences between
various classes, the origin of their activity is attributed to a combination of
emitting components, surrounding an accreting Super Massive Black Hole, in the
so called Unified Model. Currently, the execution of sky surveys, with
instruments operating at various frequencies, provides the possibility to
detect and to investigate the properties of AGNs on very large statistical
samples. Thanks to the spectroscopic surveys that allow investigation of many
objects, we have the opportunity to place new constraints on the nature and
evolution of AGNs. In this contribution we present the results obtained by
working on multi-frequency data and we discuss their relations with the
available optical spectra. We compare our findings with the AGN Unified Model
predictions, and we present a revised technique to select AGNs of different
types from other line emitting objects. We discuss the multi-frequency
properties in terms of the innermost structures of the sources.
