Introduction: The significance of radiobiology and radiotherapy for cancer treatment. Irradiation-induced damage and the DNA damage response. Cell death after irradiation: How, when and why cells die. Quantifying cell kill and cell survival. Radiation dose-response relationships. Linear energy transfer and relative biological effectiveness. Physics of radiation therapy for the radiobiologist. Tumour growth and response to radiation. Fractionation: The linear-quadratic approach. The linear-quadratic approach in clinical practice. Modified fractionation. Time factors in normal tissue responses to irradiation. The dose-rate effect. Pathogenesis of normal tissue side effects. Stem cells in radiotherapy. Normal tissue tolerance and the effect of dose inhomogeneities. The oxygen effect and therapeutic approaches to tumour hypoxia. The tumour microenvironment and cellular hypoxia responses. Combined radiotherapy and chemotherapy. Molecular targeted agents for enhancing tumour response. Biological individualisation of radiotherapy. Molecular image guided radiotherapy. Retreatment tolerance of normal tissues. Biological response modification of normal tissue reactions: Basic principles and pitfalls. Hadron therapy: The clinical aspects. Tissue response models. Second cancers after radiotherapy.