For the last few years astrophysicists and elementary particle physicists have been working jointly on the following fascinating phenomena: 1. The solar neutrino puzzle and the question: What happens to the neutrinos on their way from the sun to the earth? 2. The growing evidence that our universe is filled with about 10 times more matter than is visible and the question: What is dark matter made of? 3. The supernovae explosions and the question: What do neutrinos tell us about such explosions and vice versa? The experimental investigation of these phenomena is difficult and involves...