LUNA (Laboratory for Underground Nuclear Astrophysics) is a nuclear astrophysics experiment running at the INFN Laboratori Nazionali del Gran Sasso. Aim of the experiment is to measure the cross-section of fusion reactions that take place inside the stars and that, in the past, dominated the Big Bang Nucleosynthesis. The low value of these cross-sections, in the astrophysical range of energies, prevents any kind of measurements at the Earth's surface. On the other hand, the low background of the LNGS underground laboratory allows LUNA to investigate these reactions at energies of astrophysical interest without the necessity of an extrapolation from the highest energies. An overview on the main results obtained by LUNA in the last twenty years and their astrophysical consequences will be discussed.