Recent advances in our understanding of the molecular events leading to the development of normal tissue complications after radiotherapy has led to an effort to identify biological markers that could identify patients at increased or decreased risk for treatment related injury. The goal of this effort is to improve the therapeutic ratio and enable physicians to optimize therapy for individual patients. In radiotherapy of the thoracic region, the lung is one of the most critical dose-limiting organs. This review briefly introduces the mechanisms of radiation-induced lung injury and gives a summary of clinical research focused on evaluating changes in biological markers before, during, and after radiation therapy of the thorax.