Ultrasound of the breast is comfortable, widely available, and does not involve ionizing radiation. Indications for a breast ultrasound study include evaluation of a palpable lump, further evaluation of a mammographic or MRI abnormality, screening among women who are at increased cancer risk, and to provide guidance for a percutaneous biopsy. This chapter provides an overview of breast ultrasound, which is a key modality for diagnostic evaluation and, increasingly, for supplemental screening. Topics that will be covered here include the breast anatomy on ultrasound, imaging techniques, medical physics associated with breast ultrasound (including spatial compounding, Doppler imaging, and harmonic imaging), and clinical indications.