Radiation biology has had an important impact on clinical radiation therapy by providing a rationale for implementation of new treatment strategies and for clinical concepts or practices thereby increasing their acceptance. The observed rather narrow range of D0 and n values for mammalian cells contributed to successful trials of radiation treatment of several "radiation-resistant" tumors, e.g., carcinoma of prostate, color-rectum, and sarcoma of soft tissue. Attention of clinicians was forcibly directed to assessment of local results (local failure, treatment complications) and not merely survival at 5 years by the extensive literature of cell survival curves (in vivo and in vitro) and dose-response assays on normal and tumor tissues. Upon these same laboratory results a scientific rationale was developed for use of shrinking field technique, low dose for subclinical disease, and the combination of moderate dose radiation therapy and conservative surgery. The entire area of clinical research into altered dose fractionation schedules is based upon research on cell proliferation kinetics and repair of radiation damage. The understanding that the time for complete regression of tumor depends not only upon cell kill but also on the pattern of cell proliferation of the progeny of lethally irradiated cells and the abundance of stroma provided a basis for accepting patients with slowly responding tumors for treatment. There remains a wide field of need in research in this area as even today a large proportion of patients who die of cancer die with their cancer uncontrolled at the primary site.