The results of clinical investigation suggest that MR imaging can provide clinically important information that cannot be obtained with conventional imaging methods, and that this modality will, in the future, be an invaluable adjunctive breast imaging tool just as breast ultrasound is today. MR imaging appears to be the most accurate method for the detection of implant failure, and although it is the most costly of the available implant imaging techniques, it may be the study of choice when there is a question of implant integrity that cannot be answered with conventional methods. MR imaging as a method to detect, diagnose, and stage breast cancer remains in the investigational stage. The specificity of MR imaging appears limited because of the overlap in the enhancement kinetics and morphologic appearance of benign and malignant lesions. In selected cases, the identification of certain morphologic features, such as internal septations or the absence of enhancement, may be used to classify a lesion as benign, offering an alternative to percutaneous or excisional biopsy. MR imaging appears to be very sensitive for the visualization of both invasive carcinoma and DCIS. Perhaps most important, MR imaging can detect invasive and noninvasive breast carcinoma that is both mammographically and clinically occult, offering the potential for more accurate breast cancer staging and optimized treatment planning. MR imaging is emerging as perhaps the most promising imaging modality for breast cancer detection to date. Published results, however, are from studies with relatively small numbers of patients. The results of these studies should be validated in a large-scale clinical trial before MR imaging is implemented clinically, outside of research settings. This type of clinical investigation is needed to define the technical requirements for optimal imaging, to define interpretation criteria, to develop accurate MR imaging guided localization and biopsy systems, to define the clinical indications for which MR imaging should be used as an adjunct to conventional imaging methods, and to address the issue of cost-effectiveness. One such trial, an international, multi-institutional study funded by the National Cancer Institute, is presently underway.