Hardly anything is known about the properties of the neutrinos in spite of the fact, that they had been introduced into physics some 67 years ago. The Sun is a particular strong source of neutrinos and for this reason has been used for pertinent experiments. At first such particles were studied at higher energies, which provide larger cross sections. It now became possible by employing Gallium to study neutrinos also at the lower energies, which are the prime source for the solar energy. It is in this context, that the GALLEX project has been carried out by an international collaboration at the Gran Sasso Laboratory in Italy. The lecture will describe the theoretical and experimental features of this project.