The status of the axillary nodes is the strongest known prognostic variable in patients with early breast cancer, and is routinely used in planning postoperative therapy. Conventional axillary lymph node dissection is limited by sampling error and potential morbidity. Sentinel node techniques have revolutionized the management of axillary nodes. Accurate identification and focused histologic evaluation of the sentinel node allow accurate prediction of the tumor status of other axillary nodes, thereby avoiding the morbidity and expense of a complete axillary dissection in node-negative patients. Radiotracer techniques play an important role in the preoperative and intraoperative localization of the sentinel nodes. Optimal localization of the sentinel node requires the use of both preoperative lymphoscintigraphy and intraoperative radiosensitive probes. Lymphoscintigraphy also identifies patients with lymphatic drainage to sites other than the axilla, thereby allowing more appropriate treatment and follow-up in this subset of patients. Procedures for localizing sentinel nodes require an understanding of the kinetics of the radiopharmaceuticals or other tracers used and the detection devices employed in each institution. Both surgical and nuclear medicine personnel should understand these principles, and close cooperation between surgeons, nuclear medicine physicians, and pathologists is essential for the application of sentinel node techniques.