Across several series, the sensitivity of sonographically guided 14-gauge core biopsy is 95%, and the repeat biopsy rate averages 11%. Success of stereotactic biopsy requires obtaining larger volumes of tissue when sampling calcifications; use of vacuum-assisted devices results in comparable sensitivities. For MR imaging-guided percutaneous biopsy,success rates of 95% to 99% have been achieved. Independent of guidance method or the amounts of tissue acquired, the following diagnoses on percutaneous biopsy should generally prompt excision: atypical ductal hyperplasia, lobular neoplasia, radial sclerosing lesions, benign and atypical papillary lesions, and possible phyllodes tumor. Mucocele-like lesions may merit excision. Columnar alteration without atypia probably does not require excision, although further study is needed.