Radiation therapy is mostly a computer-based technique. From the first days of using radiation to treat cancer, it was obvious that automatic procedures are important, because implication of technicians was dangerous. All kinds of radiation therapy have evolved over the last decades because of the use of computers. New imaging techniques like computed tomagraphy and magnetic resonance imaging, new treatment machines like accelerators and afterloaders, and treatment planning software have been based totally on the evolution of computers. The whole procedure of radiation therapy is now more safe and accurate after the use of new imaging modalities and simulators, three-dimensional algorithms for dose calculation, and various optimization methods.