In patients with primary melanoma and breast cancer, lymph node status is often a strong predictor of outcome and influences the course of treatment a patient may follow after surgery. In an effort to reduce the morbidity and costs of detection of lymph node metastases, surgical oncologists have developed a method by which the sentinel lymph node (SLN; the first node in a draining basin) is identified intraoperatively and removed. This technique, called sentinel node biopsy, has extremely high negative predictive values for melanoma metastases and breast cancer metastases. The two largest trials for melanoma, Morton, et al (2005) and Rossi, et al (2006), reported false negative rates of 6.3% and 14.7%, respectively. Morton, et al (2006), in perhaps the most mature trial reported to date, showed a false negative rate of 3.4%. There is growing evidence that sentinel node biopsy will have a significant impact on the management of melanoma. Sentinel node biopsy also has extremely high negative predictive values for breast cancer metastases; the false-negative rates range from 0% to 9%. There is growing evidence that sentinel node biopsy will have a significant impact on the management of breast cancer. Although the survival and local recurrence studies have yet to be completed, the technique has emerged into common practice.