The interactions occurring during the irradiation of matter are discussed briefly. The effect of irradiation of cellulose is being especially treated and the chemical mechanism, the reaction kinetics, and possible side reactions are being outlined. On the basis of results it is shown that the degradation occurs statistically throughout the system. This accounts for the fact that the destruction of all active sites created by irradiation is only possible under conditions of altering the crystal modification of the cellulose. Side reactions, such as oxidative changes, are to be expected especially when the irradiation is performed with radiation of low intensity over longer periods. The irradiation of pulp is not accompanied by adverse effects on pulp properties as long as the irradiation does not exceed 5 Megarad.