Breast masses in children and adolescents are uncommon, and the spectrum of pediatric breast masses is predominantly benign and different from that in adults. Knowledge of the clinical presentation and imaging features of the various stages of normal development and mass-forming lesions in the pediatric breast can guide a tailored imaging approach and help the radiologist make a definitive diagnosis. Breast development begins during fetal gestation along the embryologic milk lines and continues through puberty as the breast matures through the Tanner stages of development. Normal and developmental variants and benign neoplastic and nonneoplastic lesions in the pediatric breast are common causes of concern. Malignant breast masses in children are rare and are more often due to metastasis than primary breast cancer. When clinically warranted, US is the mainstay for imaging the pediatric breast and requires careful correlation of sonographic findings with patient age and history. Breast MRI can be used to further characterize lesions and evaluate the extent of disease. Biopsy should be considered only for suspicious findings and must be weighed against the risk of iatrogenic injury to the developing breast. Given that the majority of mass-forming lesions in the pediatric breast are benign, the diagnostic and management approach should emphasize "first do no harm." Knowledge of the imaging appearance of normal breast development and the spectrum of benign and malignant pediatric breast masses is necessary to make the correct diagnosis. © RSNA, 2022.