Radiation therapy dose distributions have become more focused, oen utilizing sharp dose gradients to deliver high doses to target volumes while sparing nearby normal structures. Hence, knowledge of the tumor’s location is critical during treatment to optimize therapy and minimize complications. Multiple factors aect the location of a target volume, including daily set-up variations that occur between treatment days (interfraction errors) and changes that occur while the patient is undergoing daily treatment (intrafraction motion). ese uncertainties can be large enough to potentially compromise the ability to deliver a curative dose of radiation. Optical and image-guided radiation therapy systems have played a signicant role in improving the precision of patient treatment. Optical and remote monitoring systems are attractive for image-guided radiation treatment because they can make fast, accurate 3D measurements to verify patient’s position in real time without coming into contact with the patient. Furthermore, these systems require no ionizing radiation to make these measurements. is chapter reviews the concepts, technology, and clinical applications of optical tracing and remote monitoring systems currently in use for image-guided radiation therapy.