For many tumour types, chemoradiotherapy is now the standard of care. Concomitant administration seems to improve the results over sequential schedules in many clinical situations. A lot of mechanisms of interaction between drugs and radiation have been described, according to the mechanism of cytotoxicity of the compound. Modifications of radio-induced DNA damages or of their repair have been involved for cisplatin or 5-fluoro-uracil. Synchronisation or cytokinetic cooperation is also usually described, particularly for taxanes. At the tissular level, reoxygenation of the tumour or inhibition of the proliferation could be obtained with different drugs, and today with targeted therapies. The study of these interactions in vitro or in vivo could help the physician in selecting the best drugs to use in conjunction with irradiation and in designing his clinical protocols.