For decades, radiation therapy has constituted one of the main forms of therapy for cancer, and has undergone rapid technical development. New techniques in radiotherapy make it possible to treat a tumor with larger doses than before, while at the same time reducing damage to healthy tissues. The development in imaging techniques has allowed a more precise delimitation of tumors and monitoring of the efficacy of therapy. Three-dimensional tumor delimitation and dose calculation have long been the standard in radiation therapy. Four-dimensional and functional imaging in defining the target area for radiation therapy is becoming part of routine radiotherapy.