Breast lymphatic drainage mainly consists of the return of the outer upper part of the breast to the supraclavicular lymph nodes, and the return of the inner breast lymph nodes to the paraspinal lymph nodes. Most of the lateral and superior lymph of the breast flows back to the axillary lymph nodes along the lateral pectoralis major muscle lymphatic vessels, and then back to the supraclavicular lymph nodes. Some of the upper lateral lymphatic fluid may not flow directly to the subclavian lymph nodes through the axillary lymph nodes, and then converge to the supraclavian lymph nodes. Some of the medial breast lymph nodes flow along the intercostal lymphatic vessels to the parastomal lymph nodes and then to the supraclavicular lymph nodes. The lymphatic vessels of the two breasts have mutual transportation and anastomosis, and lymphatic fluid from one breast may flow to the other. The lymphatic fluid from the deep part of the breast can flow downward to the lymphatic ducts at the rectus abdominis muscle sheath and the hepatic falciform ligament. The metastatic mode of breast cancer is mainly lymphatic metastasis, and the direction of lymphatic drainage suggests the direction of lymphatic metastasis, e.g., the sentinel lymph nodes of breast cancer are axillary lymph nodes, and breast cancer can be metastasized to the other side through the lymphatic vessel traffic of both breasts, or metastasized to the liver.