Stereotactic ablative radiotherapy (SABR), also known in older reports as stereotactic body radiation therapy, represents an evolving and expanding radiation treatment option for many forms of local malignancy, from primary tumors to metastatic and recurrent disease. It involves the precise delivery of higher doses of external-beam radiation per treatment over a shortened treatment course compared with traditional regimens. SABR has become the standard of care for patients with medically inoperable early-stage non–small cell lung cancer (NSCLC) and is becoming a more viable option for surgical candidates with early-stage primary NSCLCs who prefer noninvasive modalities of treatment. Although SABR is being used for the treatment of primary and metastatic disease in many sites of the body, such as the central nervous system, liver, pancreas, spine metastases, and isolated nodal disease in the mediastinum and abdomen, this article focuses on treatment of NSCLC in the thorax. Specifically, this review provides the rationale, evidence, and indications for treating early-stage lung cancers with SABR.