Very high redshift galaxies studied through Gravitational Telescopes

To appear in The Fabulous Destiny of Galaxies: Bridging Past and Present, conference held in Marseille, June 2005 We present here the main results recently obtained on the identification and study of very high-z galaxies (z>7) using color-color selection techniques with deep near-infrared observations using ISAAC/VLT. Our project is taking benefit of the gravitational magnification towards the massive clusters Abell 1835 and AC 114. We discuss the global properties of our candidates (in terms of Spectral Energy Distribution, abundances, magnification factors) and the implications on the UV Luminosity Function and the Cosmic Star Formation Rate of the Universe at z>7.