Abstract: Modern advances in computers have fueled advances in imaging technologies. The improvements in imaging have in turn allowed a higher level of complexity to be incorporated into radiotherapy treatment planning systems. As a result of these changes, the delivery of radiotherapy evolved from therapy designed based primarily on plain (two dimensional) X-ray images and hand calculations to three-dimensional X-ray based images incorporating increasingly complex computer algorithms. More recently, biologic variables based on differences between tumor metabolism, tumor antigens, and normal tissues have been incorporated into the treatment process. In addition, greater awareness of the challenges to the accuracy of the treatment planning process, such as problems with set-error and organ movement, have begun to be systematically addressed, ushering in an era of socalled four-dimensional radiotherapy.