Nowadays, more than ever before, the treatment of cancer patients requires an interdisciplinary approach. Radiation therapy (RT) has become an indispensable pillar of cancer treatment early on, offering a local, curative treatment option, and symptom control in palliative cases. Due to technological advances in the last 30 years, the precision in RT planning has greatly improved, allowing the radiation oncologist to prescribe high radiation doses and thereby assuring high local tumor control while sparing healthy tissue, and thereby reducing side effects. One of the most important developments has been the widespread introduction of intensity-modulated RT (IMRT), allowing a steep dose gradient outside the target volume. But higher precision in RT delivery is only useful if the information about tumor spread is equally precise. Unlike the surgeon who can visually verify the preoperative staging and adapt the extent of the procedure accordingly, the radiation oncologist depends much more and sometimes even exclusively on imaging diagnostics to define the macroscopic (gross tumor volume [GTV]) and microscopic tumor spread (clinical tumor volume [CTV]). Therefore, molecular imaging, especially positron emission tomography (PET) and multiparametric MRI, offers exciting new possibilities to improve RT planning and monitoring. While radio-oncology has become a vast field, this chapter is supposed to give an overview of the application of molecular imaging in photon RT, which is the most common type of RT, using the example of brain tumors, head and neck cancer, lung cancer, esophageal cancer, prostate cancer, and uterine cancer.