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During its first four years of scientific observations, the Fermi Large AreaTelescope (Fermi-LAT) detected 3033 $\gamma$-ray sources above a 4$\sigma$significance level. Although most of the extra-Galactic sources are activegalactic nuclei (AGN) of the blazar class, other families of AGNs are observedtoo, while a still high fraction of detections ($\sim 30\%$) remains withuncertain association or classification. According to the currently acceptedinterpretation, the AGN $\gamma$-ray emission arises from inverse Compton (IC)scattering of low energy photons by relativistic particles confined in a jetthat, 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 tobe produced together with $\gamma$-rays, providing a much better sourcelocalization potential, we focused our attention on a sample of $\gamma$-rayBlazar Candidates of Undetermined Type (BCUs), starting a campaign of opticalspectroscopic observations. The main aims of our investigation include a censusof the AGN families that contribute to $\gamma$-ray emission and a study oftheir redshift distribution, with the subsequent implications on the intrinsicsource power. We furthermore analyze which $\gamma$-ray properties can betterconstrain the nature of the source, thus helping in the study of objects notyet associated with a reliable low frequency counterpart. In this communicationwe report on the instruments and techniques used to identify the opticalcounterparts of $\gamma$-ray sources, we give an overview on the status of ourwork, and we discuss the implications of a large scale study of $\gamma$-rayemitting AGNs.

Optical counterparts of undetermined type $\gamma$-ray Active Galactic Nuclei with blazar-like Spectral Energy Distributions