The Paul Scherrer Institute in Villigen Switzerland is the largest Research Institute in Switzerland. It has four worldclass accelerators: two Proton Cyclotrons and two Electron Accelerators. They produce beams of neutrons, pions, muons and X-rays. These are used in Materials Sciences, Energy Research and Medical Applications. A special cyclotron is dedicated to irradiate tumours with protons. Over the last 22 years more than 8'000 cancer patients have been treated this way.