Lungs larger than 3 cm in diameter are generally referred to as lung masses and are relatively likely to have lung cancer. A round or oval-shaped occupying lesion less than 3 cm in diameter is usually called a pulmonary nodule. There are three forms of lung nodules: solid nodules, partially solid nodules, and ground glass nodules. Among them, partially solid nodules have the highest probability of malignancy, followed by ground glass nodules and solid nodules. If coarse burr, vacuolar sign, air bronchial sign, pleural depression sign, etc. are depicted on CT, they are considered as malignant signs. If the nodules have smooth borders and calcifications are located in the center of the lesion or are laminar, diffuse and popcorn-like, they are most often seen as benign lesions. In addition, generally less than 5 mm in diameter, the likelihood of malignancy is small, about 1% or less; at 5∼10 mm, the likelihood of malignancy is 6% to 28%; and pulmonary nodules larger than 20 mm in diameter have an 80% probability of malignancy. Generally pulmonary nodules larger than 8 mm are favored for aggressive diagnostic and treatment measures such as biopsy or surgery if malignancy is suspected. If malignancy is not considered, or if the nodule is less than 8 mm, it is recommended to review CT in 3-6 months, and generally no significant changes in the lung nodules at 2 years of follow-up, benign probability is high. In contrast, ground glass nodules are somewhat more likely to be malignant. For pure ground glass nodules less than 6 mm, routine follow-up is not necessary; for pure ground glass nodules with nodules above 6 mm, it is recommended to determine stability in 6-12 months and to review every 2 years thereafter until 5 years. In partially solid ground glass nodules, if the solid component is greater than 6 mm, 3-6 months is recommended to determine stability, and biopsy or surgical resection is recommended only when the solid component is greater than 6 mm and lung cancer is highly suspected.