Contrast-enhanced MR imaging is being used increasingly because of its high sensitivity to breast cancer and superior ability to demonstrate the extent and distribution of disease. In addition to this direct clinical use, MR imaging in the neoadjuvant treatment setting allows exploration of its potential value in quantifying primary tumor response. The high sensitivity and staging accuracy of MR imaging may yield more accurate classification of objective tumor response using RECIST criteria than clinical examination or mammography. Functional measurements hold the promise of greater sensitivity for detecting biologic effects of targeted treatments than simple anatomic methods.