The lung is one of several moderately radiosensitive organs. Radiation-induced lung injury (RILI), including acute radiation pneumonitis and chronic radiation-induced pulmonary fibrosis, occurs most often in radiotherapy of lung cancer, esophageal cancer, and other thoracic cancers. Clinical symptoms of RILI include dry cough, shortness of breath, chest pain, fever, and even severe respiratory failure and death. The occurrence of RILI is a complex process that includes a variety of cellular and molecular interactions which ultimately leads to large fibroblast accumulation, proliferation, and differentiation, resulting in excessive extracellular matrix deposits, causing pulmonary fibrosis. The progress that has been made in recent years in the understanding of cellular and molecular mechanisms of RILI is summarized in this review.