We carried out an extensive spectroscopic campaign of blazars of unknown or uncertain redshift which are either detected in the TeV band by Cherenkov telescopes or good candidate as TeV emitters. We used the OSIRIS spectrograph at the Gran Telescopio of Canarias (GTC) at La Palma to secure high signal-to-noise ratio spectra at intermediate resolution in the range 3800-10000 Ang. These observations allow us to confirm the blazar classification, to find new redshift and/or to set stringent lower limits. The results are discussed in the context of emission modeling of TeV sources and are of importance for the characterization of the Extragalactic Background Light.