High-dose hypofractionated radiotherapy is feasible because of technological advances that allow for the precise delivery of radiation to a target volume that contains a limited amount of normal tissue. No longer constrained by the normal tissue response of large fields, very large doses of radiation can be delivered. However, with the application of high doses of radiation to very precise treatment volumes, we find ourselves wondering if the fundamental principles that govern conventional radiotherapy apply. The conventions of tumor hypoxia, reoxygenation, tumor repopulation, and intrinsic radiosensitivity are all likely to be unique or play new roles in tumor or normal tissue response. With that in mind, we highlight several facets of tumor or normal tissue response in which a better understanding of tumor biology and radiation biology could be beneficial to the application of high dose per fraction treatment modalities or in which caution should be exercised.