Positron emission tomography (PET) has initially developed as a research tool, primarily for cardiological and neurological application. In the past decade, the paramount clinical potential of this imaging procedure has been recognized, reaching its full expression in the oncological field. The number of scientific publications demonstrating the clinical accuracy of the tracer [fluorine-18]-fluoro-2-deoxy-D-glucose ([18F]FDG) in the diagnosis, grading, staging, monitoring the response to treatment and in prognostic stratification of the oncological patient is continuously expanding. Nowadays, the clinical use of PET with [18F]FDG is approved in the majority of the countries (even in those with strict health regulations) for the metabolic imaging of proliferative diseases of the lung, breast, colon, and for the adequate diagnostic and prognostic evaluation of the patient with lymphoma and melanoma. Encouraging results have been also obtained in cancers of the head and neck, ovary and musculoskeletal system. Although about 90% of clinical investigations in oncology are performed using [18F]FDG as the tracer, radiopharmaceutical research is continuously developing new and more specific tumor targeting agents, whose efficacy is being explored more and more in the wide scale.