Book excerpt: This is the first book to summarize the problems of using modern high-resolution 2-mm wave band EPR spectroscopy in an interdisciplinary field for the investigation of various condensed systems. The material is well illustrated and the applications are as diverse as possible. The main subjects included are: unique characteristics of 2-mm EPR spectroscopy and appropriate experimental techniques, dynamics and polarity of radical microenvironment in model and biological systems, and the nature of charge carriers and charge transfer mechanisms in organic polymer semiconductors.