The field of radiation oncology relies heavily on imaging modalities. From initial consultation to treatment completion, images are used to guide nearly every step of the patient encounter. Technological advances in diagnostic radiology continue to be readily incorporated into clinical practice, with adaptive and functional studies able to extract and display ever more physical and novel biological data about patients and their tumors. At the same time, no imaging technique can address all the uncertainties inherent in cancer therapy. The application, interpretation, and limitations of various imaging modalities are discussed from a radiation oncology perspective.