The sentinel lymph node procedure is currently standard care in the treatment of breast cancer. The introduction of this procedure in 1992 would not have been possible without the pioneering discoveries regarding lymph nodes and cancer. The Italian surgeon Gaspar Asellius (1581-1626) visualized the lymphatic vessels of a dog after it had been fed and shortly before dissection. At the end of the 17th century, the French anatomists Lauth and Sappey visualized the lymphatics by injecting deceased criminals with mercury. In 1858, the German pathologist Virchow (1821-1902) launched the theory that lymph nodes act as defensive barriers. He also made the first microscopical illustration ofa sentinel lymph node. Gould et al. and Cabaãs independently launched the precursors of the current modern sentinel lymph node concept in 1959 and 1977 respectively. Gould et al. were the first people to use the term "sentinel node'. Cabañas used lymphangiography for the visualisation of the sentinel lymph node. Controversies about the barrier function of the lymph nodes, the fear of skip metastasis and the difficulties of performing the Cabañas procedure, prevented a breakthrough of this concept. In 1992 Morton et al. rediscovered the valuable sentinel node biopsy concept and introduced blue dye for the investigation of patients with melanoma. The combination of lymphoscintigraphy, intra-operative gamma probe guidance and patent blue further increased the reliability of the sentinel lymph node biopsy procedure. Unnecessary lymph gland dissection procedures with considerable morbidity can be prevented by this procedure.