Sonography can be used for the accurate differentiation of many benign and malignant solid breast lesions. However, considerable experience, and close correlation with the physical examination and the mammogram, are required to do so. Sixteen sonographic signs useful in this differentiation are reviewed. The specific sonographic appearances of the most common being entities are described. Primary breast malignancies are divided into five categories according to their sonographic presentations: (1) classic neoplasms with irregular borders, echoic rims, and usually posterior shadowing, (2) small, round neoplasms with no echoic rim or posterior shadowing, (3) neoplasms with mixed or increased echogenicity, (4) cystic or intracystic carcinomas, and (5) colloid carcinomas. Methods for identification of these different types of invasive malignancy, and of in situ carcinomas, are presented. The usefulness of sonomammography is considered in specific circumstances, including evaluation of mammographic or physical findings, dense breasts, post-radiation breasts, and women under 35 years of age.