This review of clinical data from partial-breast treatment studies discusses results from a single institution's experience using brachytherapy catheters, the MammoSite balloon (Proxima Company, Alpharetta, GA), and three-dimensional external beam partial-breast treatment. Rationale, techniques, dose schedules, and available results are presented. Intra-operative single-dose radiation to the lumpectomy site is also reviewed. The National Surgical Adjuvant Breast and Bowel Project (NSABP) and Radiation Therapy Oncology Group (RTOG) cooperative groups are opening a phase III trial to compare partial-breast treatment to the standard 5- to 6-week course of whole-breast radiation. The design of this study is also presented in this review.