During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 𝛾-ray sources above a 4𝜎 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 (∼30%) remains with uncertain association or classification. According to the currently accepted interpretation, the AGN 𝛾-ray emission arises from inverse Compton (IC) scattering of low energy photons by relativistic particles confined in a jet, which, 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 𝛾-rays, providing a much better source localization potential, we focused our attention on a sample of 𝛾-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 𝛾-ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power. We furthermore analyze which 𝛾-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. Here we report on the instruments and techniques used to identify the optical counterparts of 𝛾-ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of 𝛾-ray emitting AGNs.