Molecular imaging, in particular, positron emission tomography(PET) has brought an additional dimension to management for patients with cancer. It may lead to a change in staging and treatment management. In radiotherapy, the PET or PET-CT has a crucial role in treatment planning and patient management, specifically, its use in disease staging, patient selection, treatment planning and evaluation. The purpose of radiotherapy is radical or palliative depending on the staging of tumor; thus, PET(-CT) may actually change the course of disease. The methodological and technological aspects of PET(-CT) in radiotherapy are discussed, focusing on the problem of target volume definition and tumor motion with respect to imaging and dose delivery. In addition, tissue hypoxia is an important factor in radiotherapy as a resistance factor in treatment; thus, we mention the concept of hypoxia-directed therapy.