Abstract: Optically detected magnetic resonance (ODMR) technique has shown to be a successful tool for research work on the impurities and defects in semiconductors. This method may provide detailed information about the electronic structure of the impurities and defects and the relation between them and the tr-asition processes. The technique of ODMR, especially the new developments and understanding in last few years, has been discussed in this paper. The application of ODMR to the research field of impurities and defects and radiation processes in Ⅱ-V semiconductors has been reviewed and the research work about InP;Mn and GaP:O has been discussed as examples in detail.