Breast ultrasound is a valuable adjunctive to mammography for the identification and characterization of palpable and nonpalpable abnormalities. With the advances of ultrasound techniques over the past 10 years, the role of ultrasound has evolved from differentiation of benign from malignant masses, evaluation of mammmographic abnormalities, and guidance modality of interventional procedure to preoperative evaluation of lesion extent, follow-up after operation, and screening method for high-risk women. It can also be used as a supplemental modality of a physical examination and mammography to increase diagnostic accuracy and to reduce unnecessary surgical biopsy. This article is to review the indication of breast ultrasound examinations as well as merits and limitations compared to mammmography.