Borexino is a large-scale real-time detector for sub-MeV solar-neutrino spectroscopy. It is located underground at the Laboratori Nazionali del Gran Sasso LNGS. Its main application is the flux measurement of mono-energetic (862 keV) ⁷Be neutrinos as well as pp and CNO solar-neutrino fluxes. The detection mechanism is neutrino-electron elastic scattering, ES, in ultra-pure organic liquid scintillator. Internal, external, and cosmogenic backgrounds are under detailed analysis by means of energy and position reconstruction with techniques like shape analysis of scintillation light and fast-coincidences tagging.