"This project aims at making major advances in our understanding of galaxy formation through the direct detection and spectroscopic mapping of gas outside of galaxies at high redshifts. This experiment will employ the 2nd generation ESO-VLT instrument MUSE, currently being finalised under my leadership. MUSE is a highly innovative integral field spectrograph featuring an unparalleled combination of large field of view, excellent angular resolution, and sensitivity to faint signals. The instrument will arrive at the observatory in mid 2013 and soon after begin with the buildup of a revolutionary set of “spectroscopic deep fields”, which will constitute a significant fraction of the 255 nights of observing time granted to the MUSE consortium in return for their investment.The focus of this ERC proposal is the study of the gaseous environments of galaxies, and their links to the general inter¬galactic medium. Observationally it is extremely difficult to trace the inferred gas flows, because of their low predicted surface brightnesses and filamentary geometry. Only MUSE will have the sensitivity to detect and map them directly, through the Lyα emission line of recombining hydrogen. MUSE will also provide spectroscopic (kinematic) constraints on these gas flows, which will be essential for quantitative comparisons with numerical simulations.A dedicated team will address the many challenges of this scientific endeavor in a comprehensive way. It will involve the characterization of the instrument and its systematics; the development of advanced data reduction and data analysis tools in a coordinated multidisciplinary approach, involving astronomers as well as experts in image and signal processing; and the interpretation of the results in conjunction with the latest high-resolution cosmological simulations. Upon completion, MUSICOS will deliver an entirely new view at the early stages of galaxy formation and the growth of baryonic mass in the young universe."