The variable Sun is the primary external driver of natural climate change, as evidenced by positive correlations between solar variability and various climate indicators. Even though the Sun-Earth connection has been under close scrutiny over the last two decades, a number of crucial questions remain open. Thus, in the 4th IPCC assessment report the level of our understanding of the solar influence on climate was graded as very low. One of the reasons for this is our incomplete understanding of solar irradiance variability. In particular, the magnitude (in the UV part of the spectrum) and even the phase (in the visible part of the spectrum) of the solar cycle variation remain highly controversial. Also the magnitude of solar irradiance changes on centennial and longer time scale is uncertain.The main goal of this project is to incorporate the newest information obtained by the recent high-resolution observations and magnetohydrodynamic (MHD) simulations of the solar atmosphere into SATIRE (Spectral And Total Irradiance Reconstruction), which is one of the most advanced models of solar irradiance variability.  This information will put important constraints on the relationship between the magnetic field strength and the spectral contrasts of solar active regions, crucial for successful modeling of solar irradiance variability. The improved SATIRE model will be employed to compute and release new, more reliable and more physics-based reconstructions of solar total and spectral irradiance over the Holocene. It will also be employed to analyze the unprecedentedly precise photometric time series of Sun-like stars obtained by the Kepler and Corot space missions, whose advent rekindled the interest in solar-stellar comparison. This will allow one to understand whether the photometric variabilities of Sun-like stars are governed by the same physical processes as those acting on the Sun.