By using a surgical gamma probe after peritumoral injection of a radioactive tracer, the surgeon can identify the sentinel, or first, nodal site of regional metastasis in clinically node-negative patients. In the near future, the pathological status of this node will have an important impact on the treatment strategy in breast cancer and melanoma patients. This article reviews the necessary requirements for the instrument, such as: absolute sensitivity, spectral resolution, angular sensitivity and response ratio to the radioactive source at depth. In addition, ergonomic characteristics are important. The surgeon must consider the above characteristics, as they influence operational handling. Four commercially available surgical gamma probes were subjected to laboratory tests to appraise the extent to which they fulfilled these criteria. The results for each gamma probe are summarised and discussed.<