The tumoral uptake of 2-[18F]fluoro-2-deoxy-D-glucose (18FdGlc) is based upon enhanced glycolysis. Positron-emission tomography (PET) using 18FdGlc provides the physiological and metabolic information. 18FdGlc PET has been used successfully for assessing primary tumours and metastases, prognosis, and planning and for monitoring tumour therapy as well as for early detection of recurrent tumour growth. This review summarises the uptake mechanism of 18FdGlc in benign and malignant lesions, its relation to histopathology, and its clinical value for detecting and staging primary tumours.