The sound that occurs when gas passes through a narrow airway during expiration is called croup. Airway inflammation is the most common cause of croup. In inflammation, airway mucosal edema, increased secretions and airway spasm narrow the inner diameter of the airway, increase airway resistance, and a pronounced expiratory prolongation and expiratory phase of croup will occur during expiration. In many clinical diseases such as capillary bronchitis, wheezing bronchitis, and bronchial asthma, there are signs of croup. Other causes of narrowing of the lumen of the tract are foreign bodies in the airway by mistake, such as peanuts, melons, or even toys. New organisms in the airway, such as granulomas, mediastinum, tumors, and enlarged lymph nodes compressing the airway, can produce croup. Therefore, the presence of croup should be judged in the context of past history, personal history and family history. The possibility of airway inflammation with allergic history is high, and other rare diseases should be excluded without allergic history.