Staging of a cancer of the breast reflects the anatomic extent of the tumor either at the time of diagnosis prior to treatment based on clinical, diagnostic, and biopsy information or at the time of postsurgical resection when all pathologic information obtained or studied of the resected specimen is used. In the future, chemical or biological markers may add another dimension to staging. There have been several schemes suggested for staging breast cancer, but the current recommendation of the American Joint Committee on Cancer (AJCC) jointly with the TNM Committee of the International Union against Cancer (UICC) should be universally used. More complete definitions of staging of the primary tumor (T), nodes (N), and distant spread (M), are discussed in this report. It is stressed that, obviously, the smaller a cancer and the earlier it is diagnosed has a greater influence on therapy.