Breast cancer is the most common form of cancer and second main cause of death in women in western countries. Breast-conserving therapy, consisting of lumpectomy and radiation therapy, has become the standard local treatment for T1-T2 breast tumors. There is general agreement that successful breast conservation requires complete tumor excision with a "tumor-free" or "negative" margin of resection, but the definition of a negative margin is controversial. A commonly accepted definition of adequate margins requires a 2-mm distance between ink and tumor but opinions range from the original National Surgical Adjuvant Breast and Bowel Project definition of "no ink on tumor", to a recommended width of 10 mm or more. The ability to perform real-time molecular imaging analysis of margins during surgery would clearly be a significant advance; several groups have engaged in this effort, with encouraging reports of preliminary data. Further development of such techniques promises to lead to a point at which accurate intraoperative margin evaluation may be possible and may even be combined with therapeutic interventions, using techniques such as photodynamic therapy.