The central nervous system is a common site of metastasis in patients with small cell lung cancer (SCLC) and non-small-cell lung cancer. Despite advances in combined modality therapy, intracranial relapse continues to be a common site of recurrence and a major cause of morbidity for patients with lung cancer. Prophylactic cranial irradiation (PCI) has proven to be effective in reducing the incidence of brain metastases in patients with lung cancer. Based upon results of a metaanalysis demonstrating a small improvement in overall survival, PCI is now routinely offered to patients with limited-stage SCLC after a complete or near-complete response to initial treatment. However, many questions remain unanswered regarding the optimal dose, fractionation, and toxicity of PCI in patients with limited-stage SCLC. Additionally, the role of PCI in patients with extensive-stage SCLC and locally advanced non-small-cell lung cancer is unclear. Several important collaborative group trials are under way in an attempt to further define the role of PCI in patients with lung cancer.