Stereotactic body radiation therapy (SBRT) has had a profound impact on the treatment paradigm for medically inoperable patients with stage I non–small cell lung cancer. Local control and survival outcomes from prospective collaborative trials using SBRT have been highly favorable in this challenging patient population. Further study in medically operable patients is ongoing; however, randomized trials to help answer this question have terminated early because of poor accrual. Available prospective and retrospective data are discussed for the use of SBRT with regard to the medically inoperable and operable patient populations, as well as considerations for fractionation, dose, and toxicity.