Many aspects have to be considered in planning, applying, and following up radiotherapy procedures. This field poses a major challenge for diagnostic techniques. The first goal in the definition of multimodal oncological therapy concepts is to identify the patients who benefit from a definitive or adjuvant radiotherapy. After the decision for radiotherapy, the sequential process of radiotherapy planning and application has to be initiated. The basic principle is the exploitation of the therapeutic ratio, i.e., obtaining a high tumor dose and maximal protection of the surrounding normal tissue at risk. Computed tomography (CT) is an integral part of radiotherapy planning. Aside from its diagnostic value of tumor and neighboring normal tissue identification, the radiographie density matrix of CT is the basis for calculating the three-dimensional dose distribution. Positron emission tomography (PET) is not yet established as a standard in radio-oncological concept and procedure.