Vibro-acoustography is an imaging modality that has emerged in recent years. This method is based on low-frequency harmonic vibrations induced in the object by the radiation force of ultrasound. The sound produced due to object vibration is received by an audio hydrophone and the information is mapped into an image. This paper describes application of vibro-acoustography for in vivo breast imaging. Recently, we have developed a vibro-acoustography system for in vivo breast imaging and have tested it on a number of volunteers. These results demonstrate that vibro-acoustography has suitable resolution, contrast, and signal-to-noise ratio such that soft tissue structures, cysts, and a variety of breast abnormalities and masses can be delineated in the image. Lack of image speckle in vibro-acoustography allows one to detect small microcalcifications in the image. The results have been verified using x-ray mammography. The encouraging results from in vivo experiments suggest that further development of vibro-acoustography technology may lead to a new clinical tool for breast imaging applications.