This review summarizes the results of recent ab initio molecular orbital calculations performed on DNA constituents that attempt to further our understanding of the radiation-induced damage to DNA. The results reviewed include calculations performed on the four individual DNA bases, the base pairs in gas phase and modelled aqueous phase, the deoxyribose moiety, and a portion of the sugar-phosphate backbone. The emphasis is on the electron affinities and ionization potentials of the radical species calculated under various conditions (i.e. gas phase, aqueous phase, proton transfer, base stacking), as it has been shown that the initial ion radical distribution is largely a function of these two properties. Theoretical studies of the electronic excited states of the individual bases and radioprotection of the biomolecule by various thiol compounds are also reviewed. Finally, a summary is provided to allow for further elaboration of the current model for radiation damage to DNA and to show the present advantages and limitations of ab initio theory in the investigation of such processes.