Stereotactic X-ray mammography (SM) and ultrasound (US) guidance are commonly used techniques for breast biopsy. While SM provides 3D targeting information and US provides real-time guidance, both techniques have limitations. SM is a long and uncomfortable procedure and the US guided procedure is inherently 2D, requiring a skilled physician for both safety and accuracy. We have developed a 3D US-guided biopsy system to integrate with SM. The dual modality breast biopsy system combines the advantages of both approaches with 3D US and SM targeting, near real-time 3D and real-time 2D US guidance, breast stabilisation and a confined needle trajectory. Our goal is to be able to biopsy a larger percentage of suspicious masses using ultrasound, by clarifying ambiguous structures with mammographic imaging. Using breast phantoms, we have shown that our ultrasound guided biopsy system was capable of targeting artificial lesions that were 3.2 mm in diameter, with a 96% success rate. Through this study, we also demonstrated that our system was equivalent to current clinical practice, for an in vitro biopsy task. Metal beads in known relative positions allowed us to determine the geometry of the SM system, so that stereotactic mammography could be registered to 3D US images. The target registration error was found to be 1.6 mm. This error was dominated by positioning error in the vertical direction (perpendicular to the film surface). As an adjunct to SM, we propose that 3D US could provide more complete imaging information for target identification and real-time monitoring of needle insertion, as well as providing a means for rapid confirmation of biopsy success.