Intraoperative lymphatic mapping is a rapidly emerging diagnostic approach that is revolutionizing the management of patients who have solid malignant tumors. The procedure is being performed for the most part with radiopharmaceuticals and vital blue dyes. It is widely believed that passive trapping of radioactive particles determines the sentinel lymph node (SLN) for intraoperative delineation of potential draining sites. In this article, we show that dendritic cells within the SLN actively take up and trap radioactive particles and thus define the SLN immunologically. The role of preoperative lymphoscintigraphy and the selection of the site of placement of mapping reagents for intraoperative lymphatic mapping are established for patients with melanoma. For patients with breast cancer, the role of preoperative lymphoscintigraphy is controversial. We have shown that this procedure can be performed with success in identifying SLN as hot spots 87% of the time, with 20% of the cases showing draining nodes to other basins in addition to or independent of the axilla. The use of preoperative lymphoscintigraphy for patients with breast cancer can therefore be justified. The selection of the site for placement of radiotracer and blue dye can vary for patients with breast cancer depending on the primary site of the lesion. However, based on data from our institution and others, the delivery of the mapping reagents (both radioactive tracers and blue dye) to the subareolar space may help to standardize breast cancer SLN mapping.