Positron emission tomography (PET) has become an indispensable tool in the study of cancer biology, as well as in the clinical management of patients with cancer. Quantitative measurements of tracer accumulation enable researchers to detect tumor changes much earlier than by conventional imaging modalities. ¹⁸F-FDG has been widely utilized for staging and restaging of cancer, evaluation of response to the treatment, and assessment of prognosis; however, recently target-specific PET tracers have raised even more attention. This overview discusses the current status of PET imaging in optimization of cancer therapies in preclinical and clinical studies.