Radiation is an effective therapeutic option for malignant disease. Dosing techniques have been refined to the point that radiation therapy is usually safe and well tolerated. Unfortunately, damaging effects to normal lung inside the radiation field are unavoidable, leading to the clinical syndromes “radiation pneumonitis” and “radiation fibrosis.” Onset of disease occurs 6 weeks to 6 months after radiation and includes symptoms of dyspnea, cough, chest pain, and fever. Treatment of severe cases may require corticosteroids. A chest radiograph initially shows an alveolar infiltrate inside the radiation field that later develops the typical appearance of fibrotic changes that can mimic tumor or infection. The pathogenesis involves free radical formation that directly damages cells and DNA. In addition, a diffuse inflammatory response characterized by increased lymphocytes followed by a cascade of cytokines and growth factors directs the fibrotic process. This is also seen in the contralateral untreated lung. Increased understanding of these mechanisms will suggest new strategies for improving the efficacy of radiation therapy while limiting toxic side effects.