1. Abstract The feasibility, toxicity and efficacy of Partial Brest Irradiation (PBI) are currently being studied by major cooperative groups such as the NSABP, and RTOG as well as by several major academic institutions. PBI entails definitive radiation to the lumpectomy site while minimizing radiation to the remaining breast tissue. The success of PBI is dependent in no small part to proper target localization. In radiation therapy, to treat an internal structure, external landmarks must be used to calculate the position of the internal target. These external landmarks can easily change in a non-rigid structure (e.g. breast), which would nullify their use as a reference. To address this problem, there have been recent advances in the application of ultrasound (US) technology in target localization especially with respect to the prostate. Transcutaneous tracked US emerged as the most suitable non-invasive daily prostate localization tool. In this procedure an US probe is tracked and spatially registered with respect to the treatment machine, linear accelerator. Thus if one localizes the target anatomy in the US images, then the position of the target is automatically known with respect to the linear accelerator, so that the accelerator can be correctly aimed.