The goal of all cancer therapies is to increase the therapeutic ratio, that is, to increase the probability of cure while minimizing the toxicity. Theoretically, all cancers can be cured if a sufficient dose of radiation can be delivered to all clonogens without causing undue toxicity to the patient. Obviously, this is not the case in most instances. Precise target localization, in theory, can improve the therapeutic ratio by allowing more conformal radiation delivery, enabling one to escalate the tumor dose while sparing surrounding normal tissue. In the past few decades, conventional 2-dimensional radiation therapy (RT), based on surface anatomy and bony landmarks, has given way to the computed tomography-based 3-dimensional RT, allowing the use of internal soft tissue anatomy. Great interests are now emerging in the field of radiation oncology in assessing the motion of the tumor and surrounding normal tissue, from day to day or during each treatment session, and delivering treatments that adapt to the detected motion. Commercially available 4-dimensional imaging capabilities and various strategies to deliver the radiation dose to the moving target made this new paradigm possible. In this review article, we will discuss the concept of 4-dimensional RT, particularly as it applies to lung cancer, and elaborate on the technical advances that made it possible.