Over the past couple decades, the integration of screening mammography has resulted in earlier detection of breast cancer patients. For favorable early stage patients, studies have shown that breast conserving therapy with lumpectomy followed by whole breast radiation is equivalent to mastectomy. Due to the logistical challenges of an extended six-week whole breast radiation course, the concept of treating only the portion of the breast at greatest risk of recurrence following lumpectomy has been increasingly investigated. This notion is nested in the fact that the vast majority of recurrences following surgery occur in the peri-lumpectomy region (not in other regions of the breast) and thus a more limited volume may suffice in appropriately selected patients. The reduction of normal tissues that receive high-dose radiation allows an increased daily dose to be safely delivered, thereby allowing shortening the overall treatment time to one week. This treatment algorithm has been coined accelerated partial breast irradiation or APBI and is the context of this review article.