Positron emission tomography (PET) has the potential to contribute significantly to treatment planning and to the evaluation of response to therapy in patients with cancer. For disease recurrence PET imaging provides information non-invasively. The final goal is to biologically characterize an individual patient's tumor and to predict the response to treatment at the earliest possible time. Since the development of neoadjuvant chemotherapy, PET has been proved to be the most sensitive and accurate imaging technique for early therapy response evaluation of breast tumors. Quantitative and/or semi-quantitative PET studies yield valuable information in breast cancer regarding prognosis and response to chemohormontherapy in a timely fashion. In ovarian cancer, up to now only few studies have been performed applying PET techniques for the evaluation of treatment response. These preliminary studies indicate that serial assessment of tumor metabolism by FDG-PET early during effective chemotherapy may predict subsequent response to such therapy. PET studies can be repeated without any side-effects and with low radiation exposure and results can be directly correlated with clinical laboratory data and histology. The role of PET in the context of patient management and the cost-effectiveness of this approach needs further evaluation. Therapy monitoring by PET could help to optimize neoadjuvant therapy protocols and to avoid ineffective preoperative therapy in non-responders, but this has to be proven in a larger number of patients and in different neoadjuvant settings such as chemotherapy, radiation therapy, hormone therapy or a combination of these.