Atelectasis refers to a decrease in the capacity or air content of one or more lung segments or lobes. Due to the absorption of gas in the alveoli, atelectasis is usually accompanied by a decrease in the translucency of the affected area, and the adjacent structures (bronchi, pulmonary blood vessels, and pulmonary interstitium) gather in the atelectasis area. Sometimes alveolar consolidation and compensatory emphysema in other lung tissues can be seen. Collateral gas traffic between pulmonary lobules and segments (occasionally lobes) allows a certain degree of translucency in completely blocked areas. Atelectasis can be divided into congenital and acquired. Congenital atelectasis refers to the absence of gas filling in the alveoli at birth. Clinically, there are severe dyspnea and cyanosis, and most children die of severe hypoxia after birth. The vast majority of clinical atelectasis is acquired, which is the focus of discussion in this chapter.