Radiation therapy targeting is being refined to formally accommodate location of gross disease, microscopic extension, and geometric uncertainties in the delivery process. This formalization allows the disciplines in radiation oncology practice to work collaboratively to assure target coverage while attempting to minimize toxicity in adjacent normal structures. There is a growing expectation that the precise and accurate placement of radiation dose is well in hand. The development of volumetric imaging systems integrated with the medical linear accelerator for the specific purpose of guiding therapy will permit localization and targeting of soft-tissue structures at the time of treatment. In this review, the context for development of image-guided radiation therapy is discussed, and the growing expectation of volumetric guidance is portrayed through the various technologies currently being explored in the radiation therapy community.