Understanding star formation from large to small scales is a major unsolved problem of modern astrophysics, fundamental in its own right and having a profound bearing on both galaxies and planet formation.To achieve a breakthrough in our observational and theoretical knowledge of star formation, we propose to confront numerical simulations with observations obtained with state-of-the-art instrumentation, including instruments developed by our group. We will investigate three key areas: a) Origin of filamentary molecular clouds; b) Core formation within filaments; c) Fragmentation of prestellar cores into binary stars and protoplanetary disks. Our approach is novel in that we combine observational, theoretical, and instrumental efforts within the same team, and we address star formation coherently from large to small scales, allowing us to establish links between the « macrophysics » (eg. global star formation in galaxies) and the « microphysics » of the problem (eg. formation of individual solar systems). This project will be based on extensive (sub-)millimeter continuum and spectral line studies with the Herschel Space Observatory, IRAM 30m telescope/interferometer, APEX 12m telescope, and ALMA interferometer. ORISTARS will benefit from our large « Gould Belt Survey » key project with Herschel providing the first complete census of prestellar cores and young protostars in nearby star-forming complexes. We will extensively use our lab’s magnetohydrodynamic code RAMSES. We will also take advantage of technological innovations made at CEA for the Herschel-PACS and ArTéMiS instruments to develop a large 1.2mm bolometer array/polarimeter (« Polar-Channel ») for the next-generation millimeter continuum camera at IRAM. Polar-Channel will be a new powerful tool to map polarized dust continuum emission and help clarify the role of magnetic fields in forming prestellar cores and generating the rich filamentary cloud structure revealed by our ongoing Herschel observations.