The application of the high resolution ESR technique for investigation of various biological systems has been shown. The advantages of the technique in the study of structural, conformational and dynamic characteristics have been exemplified by spin labeled human serum albumin, egg lusozyme, liposome membranes, inverted micelles, a -chymotrypsin, cotton fiber and cellulose. The polarity of microenvironment and the mechanisms of molecular mobility of the objects under study have been determined. The combination of high resolution and saturation transfer techniques has been shown to give the detailed analysis of very slow molecular motions in biological objects. Peroxide radicals in biosystems have been identified by the ESR spectra of the 2-mm waveband