Molecular imaging uses noninvasive techniques to visualize various biologic pathways and physiologic characteristics of tumors and normal tissues. In relation to radiation therapy, PET with the tracer (18)F-FDG offers a unique opportunity to refine the target volume delineation in patients with squamous cell carcinoma of the head and neck, in turn affecting dose distribution and, it is hoped, patient outcome. Even more so, in the framework of adaptive treatment and theragnostics, whereby dose distribution is adapted in space and time over the typical course of radiotherapy, molecular imaging with PET offers an elegant research avenue to further improve the therapeutic ratio. Such implementation could be of particular interest with tracers other than (18)F-FDG, such as tracers of hypoxia and proliferation.