A rapidly emerging clinical application of Positron Emission Tomography (PET) is the detection and staging of cancer. The glucose analogue tracer 2-[fluorine-18]fluoro-2-deoxy-D-glucose (18F-FDG) has been used successfully for assessing primary tumours and metastases, to indicate prognosis, planning and for monitoring of tumour therapy as well as the early detection of recurrent tumour growth. This review summarises the uptake mechanism of 18F-FDG in benign and malignant lesions, its relation to histopathology, and the potential of PET in oncology practice.