Intraoperative radiation therapy (IORT) applied to peri-tumoral tissue can play a significant role in preventing breast cancer recurrence. Approximately 80-100% of breast cancer recurrences occur at the tumor bed, thus restricting radiation to the postoperative bed may be reasonable in a select group of patients. IORT can be delivered as a boost in addition to standard external beam radiotherapy, or as a primary form of treatment. IORT can be administered via electrons generated by a linear accelerator or by a system using low-energy x-rays. Potential advantages of IORT include improved cosmesis, shorter overall treatment time, radiobiological advantages, and ability to define the tumor bed by direct visualization during surgery assuring accurate delivery of radiotherapy. IORT alone can be considered for appropriate patients with early breast cancer who may not require whole-breast radiation therapy. This review discusses patient criteria and benefits, IORT's roots, radiobiological considerations, treatment options, and device categories.