Ultrasonography (US) is of value in the evaluation and characterization of breast masses in children. Most masses represent either normal breast tissue, cysts, or fibroadenomas. Premature thelarche may be unilateral, and normal breast tissue is found at US. Cysts are commonly retroareolar; when they become infected, they appear sonographically as a complex mass. Fibroadenoma is the most frequent breast tumor in adolescent girls, and it is usually solitary, homogeneous, and hypoechoic. Malignant breast lesions are very rare in children; most are due to metastatic disease secondary to rhabdomyosarcoma, leukemia, lymphoma, and neuroblastoma, and their US appearance is nonspecific. Gynecomastia in boys can be mimicked by general obesity and pectoral hypertrophy; US is helpful in the diagnosis, especially when gynecomastia is asymmetric. Most breast lesions in children and adolescents are benign, and surgery should be avoided to prevent later deformity. US is the ideal imaging modality to evaluate breast lesions and may be used to guide a fine-needle aspiration biopsy. Color Doppler US evaluation is helpful; cysts are avascular, fibroadenomas may be avascular or hypovascular, and abscesses show peripheral increased flow. Bloody nipple discharge is more common in prepubertal patients, may occur in infants, and may be secondary to mammary ductal ectasia. Discharge commonly resolves spontaneously, and findings at US are frequently normal.