With the routine use of mammography and B-ultrasound in breast examination, the clinical report often shows that the axillary lymph nodes “show” or “swell”, which often makes patients anxious and doctors have difficulty in clarifying the nature. How to treat axillary lymph nodes objectively is a difficult clinical problem. Lymph nodes are important immune organs of the body. Normal human superficial lymph nodes are small, 0.1-2.5 cm in diameter, mostly within 0.5 cm, with a smooth, soft surface, no adhesions to surrounding tissues, capable of movement, and no pressure pain. Except for superficial lymph nodes in the submandibular, cervical, axillary and inguinal areas, lymph nodes in other areas are generally not easy to find. Enlarged lymph nodes in multiple parts of the body can be seen in certain systemic infections (e.g., tuberculosis, infectious mononucleosis), leukemia, and connective tissue diseases. Since the location of each group of lymph nodes is relatively constant and lymphatic drainage proceeds in a certain direction, receiving lymphatic vessels from a certain location and a certain organ, lymph node enlargement confined to a certain location has more diagnostic value. Lymph node enlargement is the phenomenon that lymph nodes increase in size due to internal cell proliferation or tumor cell infiltration. Lymph node enlargement is a common clinical sign. They are usually more than 1.5 cm in diameter, with changes in shape and abnormal texture. In clinical practice, the majority of lymph nodes with small size and regular morphology shown by mammography or ultrasound are normal lymph nodes and do not require special treatment and can be observed and followed up. Lymph node enlargement is generally classified into two categories: acute and chronic according to the course of the disease. If acute lymph node enlargement is mostly caused by infection of various pathogens, it is often combined with other symptoms such as fever, and the clinical manifestations are more typical. Inflammatory lymph node enlargement is often accompanied by epidermal redness and burning sensation, and if suppuration occurs, there may be fluctuation. For example, swollen axillary lymph nodes caused by acute mastitis, trauma to the upper limbs or their infectious diseases, acute simple lymphadenitis, viral infections, allergic diseases (drug fever, serum fever), etc. Chronic enlargement is mostly seen in lymph node tuberculosis, connective tissue disease and tumorigenic lymph node enlargement. Chronic lymph node enlargement is common in chronic infectious lymph node enlargement (non-specific chronic lymphadenitis, lymph node tuberculosis, filariasis, black fever, syphilis), connective tissue disease (systemic lupus erythematosus, juvenile rheumatoid arthritis), neoplastic lymph node enlargement (malignant lymphoma, malignant histiocytosis, leukemia, malignant metastases in local lymph nodes), and unexplained lymph node enlargement. Lymph node enlargement due to acute infection has obvious pressure pain and spontaneous pain because the lymph nodes enlarge rapidly in a short period of time. On the contrary, lymph node enlargement due to tumor is usually without pressure pain except for rapid growth. If the tumor tissue penetrates the lymph node peritoneum, it may invade the adjacent lymph nodes and surrounding tissues, causing the lymph nodes to adhere to each other or fix with the surrounding tissues. After determining the enlarged lymph nodes, the cause of the enlargement should be further determined. Increased peripheral blood white blood cell count and neutrophils often indicate bacterial infection. A high number of abnormal lymphocytes in the peripheral blood and an increased titer of the eosinophilic agglutination test are helpful in diagnosing infectious mononucleosis. Bone marrow aspiration is valuable in confirming the diagnosis of leukemia and malignant histiocytosis. Lymph node puncture, smear and biopsy are helpful in diagnosis. The most common site of lymphatic metastasis in breast cancer is the ipsilateral axillary lymph node. At first, the enlarged lymph nodes can be pushed, and finally they fuse with each other and become fixed. If the enlarged lymph nodes invade and compress the axillary vein, they can often cause edema in the ipsilateral upper limb; if they invade the brachial plexus nerve, they can cause shoulder pain. When examining the axillary lymph nodes, the upper limb on the affected side should be relaxed as much as possible so that the top of the axilla can be felt. If there is no lump in the breast and the first symptom is swollen lymph nodes in the axilla, it is rare to come to the clinic. When the swollen lymph nodes in the axilla are pathologically confirmed to be metastatic cancer, in addition to carefully examining the lymphatic drainage area, tumors of the lung and gastrointestinal tract should be excluded. If the pathology suggests metastatic adenocarcinoma, we should pay attention to the possibility of “occult breast cancer”. In this case, the breast lesion is usually not detected and mammography may be useful for diagnosis. If the lymph nodes are positive for hormone receptors, even if all tests fail to detect a breast lesion, it is still important to consider a tumor of breast origin. Breast cancer can metastasize to the ipsilateral axillary lymph nodes and also to the contralateral axillary lymph nodes through the intercommunication between the anterior chest wall and the internal breast lymph network, with an incidence of about 5%. In addition, advanced breast cancer may also have ipsilateral supraclavicular lymph node metastasis or even contralateral supraclavicular lymph node metastasis. It is worth noting that supraclavicular lymph node metastasis is classified as distant metastasis in the previous TNM staging. Supraclavicular lymph node metastasis is often associated with ipsilateral axillary lymph node metastasis, especially with lymph node metastasis in the apical group, but there are also cases in which symptoms and signs of supraclavicular lymph node metastasis appear earlier than those of axillary lymph node metastasis. Supraclavicular lymph node metastasis often presents as several scattered or fused masses with diameters ranging from 0.3 to 5.0 cm in the supraclavicular fossa. The initial metastatic lymph nodes are small and hard, and have a “sandy feel” when palpated. In some cases of supraclavicular lymph node metastasis, there is no obvious mass on palpation and only the supraclavicular fossa is full. However, it is not uncommon to find breast cancer with swollen supraclavicular lymph nodes. In summary, the causes of axillary lymph node enlargement include 3 aspects, namely infection, tumor and other factors. Tumorigenic includes primary tumors of the lymphatic system and metastatic tumors of the lymph nodes. Non-neoplastic includes infections, allergic reactions, connective tissue diseases, etc. When clinicians encounter the symptoms of chronic axillary lymph node enlargement, they may first fully examine whether there are simultaneous lumps, overflow and other abnormalities in the breast, and they should first rule out the possibility of breast cancer. Because clinically occult breast cancer sometimes only presents as swollen axillary lymph nodes, it is often easy to overlook. In order to rule out other lymph node metastases, lymphoma, lymph node tuberculosis, leukemia, etc. Surgical biopsy is generally recommended for those with unknown causes.