"We propose to carry out an FP7 collaborative project to provide the first ever quantitative answer to one fundamental age-old question of mankind: ‘How common are Earth analogs in our Galaxy?’. We will achieve our goal by combining the unprecedented photometric precision of NASA’s Kepler mission, the unrivalled precision of ground-based radial-velocities from the HARPS-N spectrograph, and ESA’s Gaia mission exquisitely accurate parallaxes. Ours is a transnational collaboration between European countries and the US setup to optimize the synergy between space- and ground-based data whose scientific potential can only be fully exploited when analyzed together. We ask for manpower and resources to carry out a GTO program with HARPS-N@TNG (80 nights/yr for 5 years) to measure dynamical masses of terrestrial planet candidates identified by the Kepler mission. With the unique combination of Kepler and HARPS-N data we will learn for the first time about the physics of their interiors. Some of these planets will have characteristics (masses, radii) similar to Earth, and they might be orbiting within the habitable zone of stars much like our Sun. We will search for planets similar to Earth orbiting a carefully selected sample of nearby bright solar-type stars and red M dwarfs, providing suitable candidates for spectroscopic characterization of their atmospheres with next-generation space observatories. We will combine Kepler, HARPS-N, and Gaia data products of stars in the Kepler field to underpin the occurrence rates of terrestrial planets (η) as a function of stellar properties with unprecedented accuracy. Our unique team expertise in observations and modelling of exoplanetary systems will allow us to fully exploit the potential for breakthrough science intrinsic to this cutting-edge, multi-techniques, interdisciplinary project, making the best use of data of the highest quality gathered from NASA and ESA space missions and ground-based instrumentation."