The gamma probe has long been used for scanning a region of the body. Until recently, the advent of the application of lymphatic mapping technique in melanoma and breast cancer during surgery has made the use of the intra-operative gamma probe widespread. With the ease of use and patient benefits that sentinel lymph node dissections offer, this technique has become the standard of management for melanoma as well as breast cancer patients. While surgeons at hospitals are eager to perform sentinel lymph node dissections in patients, there is a learning period for surgeons to be able to localize the sentinel lymph node with intra-operative gamma probe. Several critical characteristics for intra-operative detection systems are discussed in this article.