Lung nodules are focal round or round-like dense shadows of various sizes, with clear or blurred margins, and less than or equal to 3 cm in diameter on lung imaging. Possible causes include: pulmonary nodular disease, tuberculosis, lung cancer, lung metastases, etc. 1. Pulmonary nodular disease: pulmonary nodular disease is a non-specific granulomatous disease, which is prevalent in the lungs and lymph nodes, and can be manifested as small nodules on chest ct. 2. Tuberculosis: Mycobacterium tuberculosis into blood can form hematogenous disseminated tuberculosis, forming diffuse small nodular lesions in the lungs; in addition, tuberculosis foci can form tuberculosis balls when wrapped by the surrounding fibrous tissues, which can also be manifested as nodules in the lungs. 3. Lung cancer: lung cancer can be manifested as nodular shadow on chest ct at the early stage, and adenocarcinoma is more common, mostly infiltrative microadenocarcinoma. 4. Lung metastatic tumor: lung is a part of many malignant tumors easy to metastasize, such as osteosarcoma, soft tissue sarcoma, choriocarcinoma of uterus and so on. Most of them are manifested as multiple small nodular shadows, and with the progress of the disease, the lesions can gradually increase in size to form lumps. In addition, smoking, environmental factors, inflammation and mechanization may form small nodules in the lungs, and it is recommended that patients should go to the hospital as soon as possible to get a clear picture of the nature of the nodules, and if necessary, give targeted treatment.