New techniques and treatments in medicine occasionally take on a life of their own. Sometimes, an idea’s use is inherently obvious from the outset, sometimes not, in which case it becomes technology in search of an application. Identification of sentinel lymph nodes in breast cancer is a combination of these two options: to some extent the technique has been overused and its results overinterpreted. 10 years after the procedure’s introduction and general acceptance, a few issues still deserve further discussion despite much published work on this topic. Use of sentinel-lymph-node identification has been extended to other organ systems, yet not all the lessons learned in the breast might apply to other sites. Review of the theory behind the technique and breast cancer experience to date is important. Many of the published controversies about sentinel lymph nodes in breast cancer can be resolved by consideration of the basic ideas underlying the procedure, which is the aim of this Essay.