Radiation is the one agent among all environmental factors which may damage biological systems that is not only easily quantifiable but can also be measured with unsurpassed resolution. Its primary effects on atoms and molecules are well understood, and the secondary processes can be followed by sophisticated experimental techniques. The quantum nature of interactions and the importance of stochastic variations call for an exact mathematical description. This task is by no means simple, and presents a challenge both to the experimentalist and to the theoretician. It is hoped that a generally acceptable formalism will help to quantify radiation responses, both in radiation protection and radiation therapy, and make it possible to move from a purely empirical approach with all its fallacies to real understanding.