Over the past decade, the role of radiology in breast cancer has changed significantly because of the technical advances in mammography, greater use of ultrasonography, and the emergence of magnetic resonance imaging (MRI), as well as development of functional imaging techniques that add new dimensions to the noninvasive evaluation of patients with breast cancer. Currently, radiological evaluation of breast cancer is important not only for early detection of this disease, but also for staging, assessing certain prognostic factors, and monitoring response to treatment. This review focuses on the potential applications and limitations of positron emission tomography (PET) as a functional imaging method in breast cancer. PET with 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) has been used successfully in an increasing number of oncological applications and is considered a valuable adjunct to anatomical imaging methods, providing unique functional information for better characterization of disease. The contributions of PET to breast cancer imaging can be considered in two main categories. First, by providing qualitative and/or quantitative information, FDG-PET can aid in detection and discrimination of breast cancer in its primary location. Although, FDG-PET is certainly not necessary in every potential case of breast cancer, it can be very useful in clinically and radiologically "difficult-to-examine breasts," eg, following breast surgery. Qualitative assessment of the extent of the tumor spread provides prognostic information and allows for selection of appropriate therapy. The identification of tumor spread to the axillary nodes or to more remote nodal groups, i.e., internal mammary, or supraclavicular nodes, is probably the most practical information that qualitative FDG-PET can offer. Although it is still too early to formulate definite clinical recommendations, it appears likely that FDG-PET has the potential to reduce the number of patients requiring nodal dissection. Second, PET imaging can provide an assessment of the biological behavior of breast cancer. Quantitative and/or semi-quantitative FDG-PET yields valuable information regarding prognosis and response to therapy in a timely fashion. Preliminary studies have indicated that serial assessment of tumor metabolism by FDG-PET early during effective chemohormonotherapy may predict subsequent response to such therapy. The use of PET with the estrogen analogue 16 alpha-[18F]fluoro-17 beta-estradiol (FES) to monitor receptor function and response to hormonal therapy opens up intriguing new ways to monitor patients with breast cancer at a cellular level.