  During its first four years of scientific observations, the Fermi Large Area
Telescope (Fermi-LAT) detected 3033 $\gamma$-ray sources above a 4$\sigma$
significance level. Although most of the extra-Galactic sources are active
galactic nuclei (AGN) of the blazar class, other families of AGNs are observed
too, while a still high fraction of detections ($\sim 30\%$) remains with
uncertain association or classification. According to the currently accepted
interpretation, the AGN $\gamma$-ray emission arises from inverse Compton (IC)
scattering of low energy photons by relativistic particles confined in a jet
that, in the case of blazars, is oriented very close to our line of sight.
Taking advantage of data from radio and X-ray wavelengths, which we expect to
be produced together with $\gamma$-rays, providing a much better source
localization potential, we focused our attention on a sample of $\gamma$-ray
Blazar Candidates of Undetermined Type (BCUs), starting a campaign of optical
spectroscopic observations. The main aims of our investigation include a census
of the AGN families that contribute to $\gamma$-ray emission and a study of
their redshift distribution, with the subsequent implications on the intrinsic
source power. We furthermore analyze which $\gamma$-ray properties can better
constrain the nature of the source, thus helping in the study of objects not
yet associated with a reliable low frequency counterpart. In this communication
we report on the instruments and techniques used to identify the optical
counterparts of $\gamma$-ray sources, we give an overview on the status of our
work, and we discuss the implications of a large scale study of $\gamma$-ray
emitting AGNs.
