A. What is interstitial lung disease? Interstitial lung disease (ILD): also known as diffuse substantive lung disease. It is an umbrella term for a group of more than 200 diseases with different etiologies and different clinical features. These diseases share a common feature, namely a group of diffuse lung diseases that mainly involve the interstitial and alveolar spaces, resulting in the loss of alveolar-capillary functional units. The main bed manifestations are progressively worsening dyspnea, restrictive ventilatory dysfunction with reduced diffusion function, hypoxemia and diffuse lesions in both lungs on imaging. It eventually progresses to diffuse pulmonary fibrosis and cellular lung, leading to respiratory failure and even death. Interstitial lung diseases are a difficult group of respiratory diseases, and the diagnostic process requires the collaboration of clinical-radiological-pathologists. The diagnosis and treatment strategy of interstitial lung disease advocates: early detection, accurate diagnosis, reasonable treatment, and regular follow-up. II. How are interstitial lung diseases classified? There are many types of interstitial lung diseases. Based on the etiology, histopathological type of lung and clinical features of the disease, interstitial lung diseases are classified as follows to facilitate understanding, diagnosis and differential diagnosis. Interstitial lung diseases of known etiology: 1. occupational environmental exposures, such as allergic pneumonia (bird feeder’s lung, farmer’s lung, etc.), pulmonary silicosis, pulmonary asbestosis, hard metal lung disease, etc.; 2. connective tissue disease-related interstitial lung diseases, such as rheumatoid lung, etc.; other autoimmune diseases, such as microscopic polyangiitis, etc.; 3. lung damage due to pharmacological or other chemicals, such as methotrexate-induced lung damage 4, infectious diseases, such as viral pneumonia, human pneumocystis pneumonia; 5, radiation-related lung injury, such as radiation pneumonia; 6, tumors, such as cancerous lymphangitis; 7, familial, such as familial interstitial pneumonia, etc. 8, granulomatous diseases: such as nodular disease; Idiopathic interstitial pneumonias (IIPs): interstitial pneumonias of unknown etiology. According to the American Thoracic Society/European Respiratory Society international multidisciplinary classification of idiopathic interstitial pneumonia published in 2013, it is divided into major, rare, and unclassifiable idiopathic interstitial pneumonia. There are six types of major idiopathic interstitial pneumonia, including idiopathic pulmonary fibrosis, idiopathic nonspecific interstitial pneumonia, respiratory fine bronchitis with interstitial lung disease, desquamative interstitial pneumonia, cryptogenic mechanized pneumonia, and acute interstitial pneumonia. There are 2 rare types of idiopathic interstitial pneumonia, including idiopathic lymphocytic interstitial pneumonia and idiopathic pleural pulmonary parenchymal elastin fibroplasia. Idiopathic interstitial pneumonia that cannot be classified. Other interstitial lung diseases: diseases with independent clinical, imaging, and pathologic features. Examples include pulmonary Langerhans cell histiocytosis, alveolar proteinosis, lymphangioleiomyomatosis, eosinophilic pulmonary infiltrates, and alveolar microlithiasis.