The 21st conference in this series takes place from September 12 to 16, 2016 at the Federal Institute of Technology in Zürich. The cyclotron is a simple and efficient particle accelerator and its invention for the purpose of performing fundamental research dates back to 1929. Ernest Lawrence received the Nobel Prize for his idea in 1939. Today cyclotrons are used in a broad range of applications from large and complex facilities for basic research to highly optimized and cost effective solutions for industrial and medical applications. The series of cyclotron conferences provides a forum for the world leading experts to meet and to discuss technological and physics advancements in the field. The Paul Scherrer Institute operates two outstanding accelerator facilities that are driven by cyclotrons – the High Intensity Proton Accelerator HIPA, generating a very intense beam of 1.4 MW power, and the proton therapy facility Proscan with 3 Gantries. These facilities support research activities ranging from particle physics with intense muon beams and ultra-cold neutrons through solid state physics using thermal neutrons and surface muons to isotope production and cancer treatment with precise proton beam irradiation. A large international user community, including many researchers at PSI and ETH make ample use of these opportunities, and numerous important results have been achieved. In the 57 years history of cyclotron conferences this is the second time that the event is being held at the ETH in Zürich. We expect that on this occasion, as then in 1975, a good exchange of ideas will lead towards further development of these accelerators, providing even greater opportunities for research and applications with cyclotron based facilities.