Summary: Positron emission tomography (PET) is a new diagnostic method used in oncology. It is applied in differential diagnostics of benign versus malign tumors and/or disease relapse versus post-therapeutic changes. It also represents an important complementary method to the standard diagnostic methods such as computer tomography (CT) and/or magnetic resonance imaging (MRI). Recently PET has been employed as a tool in prediction of treatment response and in selecting patients with poor prognosis. PET can also assist in defining the target volume during radiotherapy planning. Increased utilization of PET in the oncology practice can lead to changes in treatment strategies and to improved therapeutic results.