Diffuse interstitial lung diseases (DILD) are heterogeneous set of diseases affecting the interstitium of the lung. The etiologic factors may be known or unknown. The diagnosis is based on the disease history, findings of the physical and radiological examinations, physiological and bronchoscopic findings, the cyto- and/or histopathological findings of the bronchoscopic materials and surgical lung biopsy in some selected patients. The reader will find the definition, classification, general properties and clinical approach of DILD.