Optics and radiators for RICH

An overview is given of the basic optics-design principles for Ring Imaging Cherenkov (RICH) counters in High-Energy Physics — of the earlier evolution of these principles and of the new ideas and techniques that are currently being developed. The characteristics of the different gasses, liquids and solids that are employed as Cherenkov radiators, which are of fundamental importance to the optics design of a RICH counter, are also reviewed.