Twenty years of LUNA

One of the main ingredients of nuclear astrophysics is the knowledge of the thermonuclear reactions responsible for the stellar luminosity and for the synthesis of the chemical elements. Deep underground in the Gran Sasso Laboratory the cross sections of the key reactions of the proton-proton chain and of the Carbon-Nitrogen-Oxygen (CNO) cycle have been measured right down to the energies of astrophysical interest. The main results obtained by LUNA are reviewed, and their influence on the comprehension of the properties of the neutrino and of the Sun are discussed.