The use of three-dimensional conformal external beam accelerated partial breast irradiation (3D conformal APBI) to deliver hypofractionated adjuvant radiation treatment to the region of the breast at highest risk for developing a local recurrence after lumpectomy is reviewed. Potential advantages of a 3D conformal radiation therapy approach for APBI compared to brachytherapy include improved dose homogeneity within the target volume and a non-invasive method for treatment delivery. Data from several early studies using 3D conformal APBI from New York University, William Beaumont Hospital, RTOG 0319, and Massachusetts General Hospital demonstrate technical feasibility, satisfactory early cosmetic outcome, and acceptable recurrence rates. To determine whether APBI provides equivalent local tumour control in the breast compared to conventional whole breast irradiation in the local management of early stage breast cancer, the first phase III randomized study of conventional whole breast irradiation vs. partial breast irradiation was opened conjointly by the NSABP and RTOG in March 2005. The results of this study will determine future directions in the treatment of early stage breast cancer, and, will help to define the role of APBI in breast conservation therapy.