The twentieth century witnessed the birth of the medical discipline of Radiation Oncology, beginning with the discovery and clinical application of radium in the early 1900s. Subsequently, clinical radiation oncology evolved as a major modality for the treatment of most solid cancers with the initial use of orthovoltage x-rays, to the use of supervoltage photons and electrons and, most recently, to the use of neutrons, protons, and other charged particles. In the last quarter of the century, Radiation Oncology readily adapted and integrated the use of supercomputers and new imaging modalities (CT, MRI, PET/CT) into radiation treatment planning and delivery to transition from 2-dimensional (2-D) to 3-D, and now to 4-D treatment. As we moved through the first decade of the twenty-first century, our ability to carefully conform the radiation dose to the outlines of irregularly shaped tumor or target volumes while also creating a steep radiation dose gradient to adjacent normal tissues is now becoming standard-of-care with the use of image-guided and intensity-modulated radiation therapy (IGRT and IMRT, respectively) or with the use of stereotactic radio surgery (SRS). This rapid evolution in imaging and radiation treatment technologies was only made possible by the simultaneous integration of specialty trained medical physicists and dosimetrists as essential members of Radiation Oncology departments.