Image-guided radiotherapy (IGRT) is in the midst of a strong development and implementation cycle, stimulated by pioneering work performed in Japan. We present a review of the rationale, technology, and methodology of image guidance, as well as an overview of current work in IGRT at the Massachusetts General Hospital. The technology is rapidly evolving, and synergisms between the various acquisition approaches are converging to provide unparalleled information on target and normal tissue location and motion. With these new approaches to patient localization, we expect improved clinical results to be forthcoming.