Breast cancer represents the most frequent malignant disease in women and the second leading cause of cancer death in western countries. Current available treatment includes surgery, radiation therapy, chemotherapy, and hormonal therapy. The disease is often curable when detected in early stages. Mammography is the most important screening modality; however, limitations of available procedures for the diagnosis and accurate staging of breast cancer has increased the application of metabolic imaging by positron emission tomography (PET). PET using the radiolabeled glucose analogue, F-18 fluorodeoxyglucose (FDG), enables visualization of increased glucose metabolism of malignant tissue. PET has been used successfully in an increasing number of oncological applications. It is an excellent clinical method to detect breast cancer over 1 cm in diameter and to accurately identify the extent of axillary lymph node metastases in patients with locally advanced disease. Recent reports have shown the high accuracy of FDG-PET imaging for staging of breast cancer patient by using whole-body PET imaging. The metabolic signal of tumor tissue allows for monitoring the effect of chemotherapy. FDG-PET can differentiate between responder and nonresponder early in the course of therapy. By identifying nonresponding patients, PET can help to avoid ineffective therapy and therefore, reduce toxic side effects in these patients.