Pulmonary nodules are focal lesions that are <3 cm in diameter, while those >3 cm are called masses. Observation of pulmonary nodules with CT requires looking at the size, density, and margins of the nodule, including whether the margins are clear or blurred, and whether there is lobulation or burr. The relationship between the nodule and the pleura, whether there is any pulling of the pleura, whether there is any thickening of the adjacent pleura, etc. Pulmonary nodules can be divided into three categories according to their density, namely ground glass density nodules, solid nodules and mixed density nodules, or sub-solid nodules. Usually benign nodules are mostly round or ovoid, with regular shape, smooth margins, no signs of lobulation or burr, and uniform density or with coarse calcification. Inflammatory nodules may cause thickening of the adjacent pleura, while malignant nodules are mostly irregular in shape, with lobulated edges, burrs, uniform or heterogeneous density, and rare calcification, often pulling the adjacent pleura to form the pleural depression sign, and the blood vessels around the nodule converge to the nodule to form the vascular pooling sign. Therefore, by observing the various signs and symptoms of nodules on CT, a judgment can be made on the benignity and malignancy of nodules.