The insertion devices (IDs) are instruments to produce highly brilliant synchrotron radiation (SR) to be installed in straight sections of a storage ring. Periodic magnetic fields are generated in the IDs mostly by permanent magnets, in which an electron moves along a sinusoidal orbit. The SR emitted at each period is superimposed coherently or incoherently, resulting in much higher brilliance than SR emitted at bending magnets. Characteristics of SR emitted from IDs are explained briefly to help the SR users utilize effectively the capability of SR.