Three-dimensional (3D) US is recently rapidly gaining popularity as it moves out of the research environment and into the clinical setting. Advances in two-dimensional high-frequency US transducer technology in combination with compound imaging and speckle reduction is the basis for high-quality three-dimensional volume US. Four-dimensional US is real-time 3D rendered image information. In addition to conventional 2D sonography, 3D/4D US provides new diagnostic information, allowing us to study a breast mass and the surrounding tissue in three orthogonal planes, or to obtain information about the three-dimensional vessel architecture using glass body rendering. In breast imaging, 3D US has the capacity to demonstrate lesion margins and topography, thereby helping differentiate benign masses from malignant ones. Three-dimensional US can also help determine the need for biopsy and help facilitate needle localization and guidance during biopsy. With recent advances in computer technology and display techniques, 3D US will likely play an increasingly important role in medicine.