Breast cancer continues to be one of the most common cancers in North America and Western Europe. Positron emission tomography with 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (FDG PET) represents a non-invasive functional imaging modality that is based on metabolic characteristics of malignant tumours. In breast cancer, FDG PET is more accurate than conventional methods for staging of distant metastases or local recurrences and enables early assessment of treatment response in patients undergoing primary chemotherapy. Recent data indicate a rationale for the use of FDG PET in cases of asymptomatically elevated tumour marker levels in the presence of uncertain results of conventional imaging. Despite the fact that PET cannot rule out microscopic disease, it does have particular value in providing, in a single examination, a reliable assessment of the true extent of the disease. This technique is complementary to morphological imaging for primary diagnosis, staging and re-staging. It may become the method of choice for the assessment of asymptomatic patients with elevated tumour marker levels. This method, however, cannot replace invasive procedures if microscopic disease is of clinical relevance.