Clinical radiobiology is a field of medical research concerned with the relationship between a given physical absorbed dose of radiation in the range used for cancer therapy and the resulting biological response in a human as well as with the factors that influence this relationship. What is seen in clinical practice is a broad range of doses where the probability of a specific type of radiation response increases from 0% towards 100% with increasing dose (i.e. a dose-response relationship). Although the term tolerance is frequently used in a loose sense when discussing radiotherapy toxicity, it is important to realize that there is no dose below which the complication rate is exactly zero: there is no clear-cut tolerance dose, although of course the probability of a given effect may become very low as the dose tends to zero.