The use of ultrasound to evaluate mammographically detected breast lesions has increased rather dramatically in recent years. This increase is directly related to the availability of high-frequency linear-array transducers and computer-enhanced image capabilities of modern-day ultrasound equipment. Concomitantly, breast clinicians have recognized the substantial potential for ultrasound use far beyond the simple differentiation of a lesion's cystic versus solid nature. Breast ultrasound has demonstrated its usefulness in quantitatively characterizing breast lesions so that they may be categorized into a number of descriptive groups based on their benign or malignant features. On the basis of such categorization and relative risk for malignancy, determination of the need for and guidance of fine-needle or core-needle biopsy can be made. This article reviews the current status of diagnostic and interventional ultrasound of the breast, with particular attention to indications, techniques, and limitations.