A few important concepts in radiobiology are illustred. The cell survival, the concept of the biologically effective dose, the basis of fractionation in radiotherapy are considered. Slow tumor regression after irradiation is not an indication of treatment failure, and the rate of regression is not , in general, prognostic. Dose volume histograms provide many data for predicting tumor control and side effects. The magnitude of a dose reduction in the tumor is the major determinant of decline in tumor control probability. Escalation of dose to hypoxic foci may be beneficial. Basic knowledge of these concepts is essential for daily radiotherapy practice and for all radiation oncologists.