SummaryUltrasound examination of the breast has recently come to maturity with the availability of high-frequency, hand-held transducers. It is now routinely used in breast imaging centers as an essential complement to physical examination and mammography not only to differentiate cystic from solid masses but also to further characterize solid masses. New diagnostic parameters include the tumor's shape, vascularity, and mobility. Although sonography can detect some carcinomas missed by mammography, its inability to demonstrate microcalcifications and minimal carcinomas and its dependence on operator skill are insurmountable obstacles that prevent its use for cancer screening. New applications of breast ultrasound in cancer management include the refinement of cancer staging through the detection of nonpalpable regional lymph node metastases, the evaluation of response to chemotherapy of locally advanced carcinomas, and the early detection of local recurrences following conservative treatment. Because of its unique real-time capability, sonography is the most accurate and convenient technique for guiding needle biopsy or localization of any nonpalpable breast mass or lymph node visible on sonograms.