After the diagnosis of a breast cancer has been established, an experienced clinician should inform the patient in a compassionate manner and reffer her to a breast care expert, who will provide more information and support and help establishing new contact. Knowing the extent of disease is extremely important in order to determine the most adequate management plan. The prognosis in breast cancer is highly related to the stage of the disease at presentation.

The purpose of staging is to determine which patients are suitable for a radical, potentially curative combination of regional treatments and appropriate adjuvant systemic therapy from the patients with metastatic disease, who are usually and primarily treated with systemic therapy. The clinical examination is the first way of providing some amount of information of the size of the lesion, the presence of the involvement of the thoracic wall or the skin, the presence of a spontaneous nipple discharge and whether the cancer presents inflammatory features.

For an  easier management, breast cancer can be classified, based on the extent of the disease into three separate groups: early or operable breast cancer, locally advanced disease, and metastatic breast cancer.

A more complex and elaborate classification is the one established by UICC (Union Internationale Contre le Cancre), known as TNM (Tumour / Node/ Metastasis). This classification, however is considered by some clinicians to be "not ideally suited to breast cancer". According to TNM classification breast cancer stages are defined by three parameters: tumor, lymph nodes and metastasis.

Depending on the tumor (T), breast cancer staging is classified in: TX (primary tumor which cannot be assessed, T0 (there is no evidence of primary tumor), Tis (tumor in situ), T1 (tumor of up to 2 cm in diameter), T2 (tumor diameter is between 2 and 5 cm), T3 (tumor larger than 5 cm) and T4 (tumor of any size with direct extension to toracic wall or skin or inflammatory carcinoma).

Depending on the involvement of regional lymph nodes the breast cancer may be characterised as: NX (the lymph nodes cannot be assessed), N0 (with no regional lymph node metastasis), N1 (metastasis to the ipsilateral axillary node / nodes), N2 (metastasis to ipsilateral axillary node/ nodes fixed to one another or to other adiacent structures, or clinically apparent ipsilateral internal mammary nodes with no evidence of axillary node metastasis) and N3 (metastasis to the  ipsilateral infraclavicular node / nodes or ipsilateral internal mammary lymph node/ nodes associated with axillary lymph node metastasis, or metastasis in ipsilateral supraclavicular lymph node / nodes.

M, or distant metastasis is another important parameter in evaluating the stage of a breast cancer. Depending on it, breast cancer may be evaluated as MX (in which distant metastasis cannot be assessed) M0 (there is no distant metastasis) or M1(in which distant metastasis is present).

Depending on these parameters the staging of breast cancer can be summarized as: stage 0 (carcinoma in situ, intraductal carcinoma, Paget’s disease of  the nipple with no tumor or lobular carcinoma in situ), Stage I (in which the tumor does not involve axilar lymph nodes) stage IIa (T2, N0, or T 1 and N positive) stage IIb (T3, N0, or T 2–5 cm and N positive (<4 axillary ipsilateral nodes), stage IIIA (T 3, N positive, or T2 with 4 or more axillary nodes involved), stage IIIB (T4 spread to less than 10 axillary nodes), stage IIIC (in whic the tumor  has affected more than 10 axillary nodes, one  or more supraclavicular or infraclavicular lymph nodes, or internal mammary nodes) and stage IV (defined by any form of distal metastasis).