There have been important developments in understanding the difference in pathogenesis and clinical significance between acute or sporadic pneumonitis and late radiation fibrosis. Corticosteroid therapy and other forms of therapy are useful in the treatment of acute pneumonitis. Late radiation fibrosis is refractory to treatment; therefore, minimizing the likelihood of developing it is particularly important. Baseline lung assessments are appropriate in patients who are clinically at risk. A new development is the use of the DVH to compare radiation treatment plans to minimize the volume of normal lung irradiated in patients who are at risk. It is hoped that the study of mechanisms that lead to the development of radiation fibrosis will point the way to possible future therapies. Patients who are included in studies of novel irradiation treatments for lung cancer need, in particular, to be monitored for late radiation lung toxicity.