We summarize the main results obtained recently by our group on the identification and study of very high-z galaxies (z>7) using lensing clusters as natural gravitational telescopes. A description of our pilot survey with ISAAC/VLT is presented, aimed at the spectroscopic confirmation of z>7 candidate galaxies photometrically selected from deep near-IR, HST and optical ground-based imaging. The first results issued from this survey are discussed, in particular the global photometric properties of our high-z candidates, and the implications for the global star formation rate at very high-z.