Stage 0 is used to describe non-invasive breast cancers, such as DCIS (ductal carcinoma in situ). In stage 0, there is no evidence of cancer cells or non-cancerous abnormal cells spreading to other parts of the breast in which they started, or getting through to or invading neighboring normal tissue.

Stage 1 describes invasive breast cancer (cancer cells are breaking through to or invading normal surrounding breast tissue) Stage I is divided into subcategories known as 1A and 1B.

Stage 2 breast cancer is still in the earlier stages, but there is evidence that the cancer has begun to grow or spread. It is still contained to the breast area and can generally be treated very effectively. Stage 2 is divided into subcategories known as 2A and 2B.

Stage 3 breast cancer is considered advanced cancer with evidence of cancer invading surrounding tissues near the breast. Stage 3 is divided into subcategories known as 3A, 3B, and 3C.

Breast cancer stage 4 describes invasive breast cancer that has spread beyond the breast and nearby lymph nodes to other organs of the body, such as the lungs, skin, bones, liver, or brain.