Knowledge of appropriate ultrasonographic (US) techniques and the US appearance of the normal breast and specific lesions is essential for successful application of breast US. The highest quality breast sonograms are achieved with 5-10-MHz linear transducers, imaging the lesion within the transducer focal zone, and relatively steep time-gain compensation curves. On sonograms, the fat in normal breast parenchyma is hypoechoic, fibrous tissue is echogenic, and glandular tissue is intermediate in echogenicity. Some normal structures can simulate masses, including the anterior costochondral junction when imaged in cross section and fat lobules outlined by Cooper ligaments when imaged in a perpendicular plane. Cysts have an anechoic interior, sharp margins, and posterior acoustic enhancement. Benign solid lesions are usually hypoechoic but variable in US appearance, which can overlap with that of complicated cysts. The classic US appearance of breast carcinoma is a hypoechoic mass with inhomogeneous internal echoes, irregular margins, and variable acoustic shadowing, although carcinoma can appear well circumscribed or have posterior acoustic shadowing in some cases. The most important function of breast US is differentiating a cyst from a solid lesion. US is also useful for evaluating a palpable mass in either young patients (< 30 years old) or those with dense breasts and negative mammographic results.