Molecular imaging with positron emission tomography (PET) using tumour-targeting radiopharmaceuticals has gained widespread acceptance in oncology with numerous clinical applications. The hybrid imaging modality PET/CT (computed tomography) allows simultaneous assessment of both molecular and morphologic information. Therefore, PET/CT represents an efficient tool for comprehensive whole-body staging and restaging within single imaging session. In oncology, the glucose analogue 18-F-fluorodeoxyglucose (FDG) is the most widely used PET/CT radiopharmaceutical in clinical routine. FDG PET and FDG PET/CT have been used for staging and restaging of tumour patients in numerous studies. This chapter will discuss the applications and primary indications of FDG PET/CT in oncology with particular emphasis on lung cancer, lymphoma, head and neck cancer, melanoma and breast cancer, among other tumour entities. A review of the current literature is presented, focusing on primary diagnosis, staging and detection of recurrent disease. Besides its established role in diagnosis, staging and restaging of disease in oncology, there is growing evidence that FDG PET/CT is valuable in assessment of therapy response, potentially influencing treatment planning and therapeutic decision making by evaluating tumour control, which will also be discussed in this chapter.