Trapped free radicals are very often responsible for postirradiation effects. Such effects are, for example, thermal luminescence, bleaching, sensitivity to oxidation, depoly merization, crosslinking, and degradation. The identification and measurement of tree radicals produced by irradiation can most readily be carried out by making use ot electron spin resonance (ESR). In general, the detection of simple atoms requires operation at low temperature, often as low as 4.2 K, that is, at liquid helium temperatures. At such temperatures, atoms of hydrogen, deuterium, and nitrogen and also alkyl radicals have been found by ESR methods. On the other hand, quite high concentrations of polymer radicals with long lifetimes are found at room temperature. While the interpretation of the ESR spectra for simple atoms is relatively simple, that of the more complex polymer radicals is much more complicated. In polvmeric systems, steric elfects and hindrance of rotation about bonds influence the various interactions to such an extent that comparisons with model compounds of low molecular size are of little value in establishing radical structures. With all polymeric systems studied thus far, full understanding and interpretation of the ESR spectra and their behavior with temperature have not yet been achieved. Nevertheless, the mechanisms of the chemical changes induced by radiation, as well as the influence of the polymeric environment on the radical species and their ESR spectra, are gradually being clarified. Studies such as those reported here on polystyrenes and fluorocarbon polymers have greatly aided in understanding the chemical basis lor physical changes in polymers.