Abstract Solar neutrinos have been detected over the past 40 years, providing the first evidence of physics beyond the Standard Model for elementary particle interactions. The hypothesis of neutrino oscillations has offered a solution to the long-standing solar neutrino problem (missing solar neutrinos). The Solar Standard Model is a fundamental ingredient in the interpretation of the solar neutrino measurements. New determinations of solar metal abundances caused the Solar Standard Model predictions to be in conflict with helioseismological measurements. In this framework, Borexino has detected for the first time sub-MeV solar neutrinos in real time by means of a 100 ton fiducial target mass of organic liquid scintillator. The direct measurement of 7 Be solar neutrinos performed by the Borexino experiment at the Gran Sasso underground laboratory allows us to probe the Solar Standard Model assuming the neutrino oscillation scenario. At present, Borexino has measured 7 Be solar neutrinos at the level of 10%. In the near future, a measurement at the level of 5%–3% could be achieved. Borexino also aims to detect pep solar neutrinos after tagging of 11 C, a cosmogenic background. Besides solar neutrinos, Borexino can detect electron anti-neutrinos from long-lived radioactive isotopes from the interior of the Earth. These neutrinos, so-called geo-neutrinos, have been observed in Borexino with a high confidence level. Due to the low detection threshold, about 200 keV, Borexino is also able to detect neutrinos from core collapse supernovae mainly by means of the inverse-beta decay and neutrino–proton elastic scattering. This latter interaction channel is a unique feature of Borexino at present. In this paper, both a description of the Borexino detector and a discussion of its physics results are presented.