Breast cancer and melanoma metastasize predominantly via the lymphatic route. It has long been known that invasion into one or a few nodes draining the primary tumour, the sentinel nodes (SN), is the most important, early sign of dissemination. If no malignant cells are detected in the SN, dissemination is unlikely to be expected. For the last 10 years SN biopsy has become an important tool in staging cancers. Two kinds of tracers are used for SN detection: The blue dye, injected during operation, and radioactively labelled colloid, injected before operation. The lymphatic drainage can then be mapped by following the blue dye by visual inspection during the operation, and with gamma camera imaging before and probe detection during the operation. The variations in the tracers used, and the injection and imaging techniques are discussed. The pathologic examination has also undergone a rapid evolution with more detailed analysis including immunohistochemistry. The use of the SN technique has quickly spread worldwide for melanoma and breast cancer but is also being tested in several other cancers. Reports on the influence on morbidity and mortality reduction are becoming increasingly convincing. The near future of SN examination is finally briefly outlined.