Non-small cell lung cancer (NSCLC) is the second most common solid malignancy in the United States of America, and the leading cause of cancer-related mortality. Nearly 15% of patients present with early-stage disease, for which the standard of care is lobectomy. However, the median age at diagnosis ranges from 65 to 74 years, and many patients have significant comorbidities that preclude surgical treatment. Previously, the standard of care for these patients was definitive radiotherapy (RT) with conventional fractionation (i.e., 1.8-2 Gy fractions delivered over 30-33 treatment sessions). Local control following this treatment ranged from 30 to 70%, and overall survival was poor at 15 to 30%. Over the years, advances in RT technology, imaging, planning, and RT delivery platforms have allowed the advent of stereotactic body radiotherapy (SBRT), in which large doses of radiation are delivered to a given target volume over a small number of fractions, while sparing adjacent organs-at-risk. Based on extremely promising local control rates in prospective studies, SBRT has quickly become the standard of care for treating early-stage NSCLC in medically inoperable patients. Here, we review the historical context of RT for NSCLC, provide a brief of overview of the radiobiological rationale for and the medical physics aspects of SBRT, and review the clinical evidence for its efficacy. We then briefly discuss future directions, including identifying the optimal dose/fractionation regimen and exploring the treatment of centrally-located tumors. Finally, we discuss the available data regarding the use of SBRT for medically operable patients.