Positron emission tomography (PET) is a medical imaging method belonging to the realm of Nuclear Medicine. It has been since the sixties as a clinical research tool, but during the late nineties it became widely utilized in clinical practice too. PET technique requires special radioactive isotopes, which may be generated only in particle accelerators (cyclotrons) and their transport is limited owing to the short physical half-life. PET/CT was born from the combination of PET and CT (computer tomography). The first combined PET/CT scanner began to function in 1998 and the method has been used in the clinical practice since 2001. It is a hybrid (multi-modality) medical imaging equipment which can provide anatomical (CT) and functional information (PET) simultaneously. PET/CT imaging has gained clinical acceptance mainly in Oncology – owing to the attributes of the most frequently used PET tracer – and in a lesser extent in Neurology and Cardiology. For the last four years more than 700 PET/CT were installed all over the world and nowadays about 1300 PET or PET/CT function in the USA, as well as another 1000 work in Europe. The number of examinations is near to 6 million worldwide. In the following we will review the basics of the method, its indications in Oncology, furthermore the most common causes of false positive and false negative results.