[en] Whereas radiotherapy has become one of the key treatments against cancer, the issue of a tolerance threshold is important in order to avoid unwanted side effects, and as the increased probability to develop a cancer corresponds to a form of susceptibility to ionising radiations, this document, based on several publications, discusses this issue of susceptibility. Several topics are more particularly addressed: cell death, survival of damaged cells and tissue reaction, signal and repair of DNA damages, and other approaches (apoptosis, single nucleotide polymorphisms or SNP of DNA repair genes, gene-candidate approach, proteomics, number of clonogenic cells). Other aspects are finally discussed (animal sensitivity, human radiobiology studies).