AbstractIn large‐capacity, high‐speed and long‐distance optical communication systems, an optical isolator is one of the key devices for stable laser diode operation.This paper discusses first the required characteristics of an optical isolator, and then features of its various types based on connection with surrounding optical circuit are described. Next, the stage of the art of the magnetooptical materials required for optical isolators and the magnetic circuit design are reviewed; and, finally, recent developments of optical isolator devices are rep