Pulmonary nodules are often seen on chest CT exams or physical examination reports, so what are they? A pulmonary nodule (isolated pulmonary nodule) is usually a solitary, round or round-like nodular lesion in the lung ≤3.0 cm in diameter. Lesions >3.0cm in diameter are called masses and have a malignancy rate of over 90%. Lung nodules can be lung cancer or benign lung tumors such as malignant pseudotumors, inflammatory pseudotumors, pulmonary metastases, and sclerosing hemangiomas. The classification of pulmonary nodules is roughly divided into: micro nodules (3.0-5.0mm), small nodules (5.0-10.0mm), and nodules (10.0-30.0mm) based on their diameter; and solid nodules, partially solid (mixed ground glass density) nodules, and non-solid (completely ground glass density) nodules based on their density on CT. The benignity or malignancy of pulmonary nodules is closely related to the size of the nodule and the age of the patient. Pulmonary nodules are most often seen in middle-aged and elderly people and usually have no symptoms such as cough, sputum and chest pain. The size of the nodule, the internal signs of the nodule, the signs of the nodule tumor itself, the paraneoplastic signs of the nodule, and the growth rate of the nodule should be analyzed. Therefore, making a definite diagnosis of pulmonary nodules is the focus, difficulty and hot spot of clinical research at present. The clinical procedures for the management of pulmonary nodules are: (1) detection of pulmonary nodules; (2) localization and diagnosis; (3) qualitative diagnosis; and (4) surgery. The following signs are generally considered to indicate a high probability of benign nodules: no change in nodule size or even a decrease in size at 3-month, 6-month or even one-year follow-up; calcified foci or popping corn-like changes in the nodules; low peak enhancement or mainly circumferential enhancement on CT dynamic enhancement scans; smooth nodule margins; and satellite signs. The following signs are highly suggestive of malignant nodules: enlargement or increase in nodules on short-term (3-6 months) follow-up; inhomogeneous density in nodules; vacuolation or inflatable bronchial signs; CT dynamic enhancement values of 20-60 Hu; deep lobar, spine, burr or bronchovascular cluster signs. For malignant pulmonary nodules, surgical resection is required; benign pulmonary nodules can be left untreated; those difficult to identify need to be followed up and observed.