Diffuse interstitial lung diseases (DILDs) are a heterogeneous group of non-neoplastic, non-infectious disorders resulting from damage to the lung parenchyma, and present with similar clinical features. There has been difficulty in the classification and diagnosis of DILDs because they contain more than 200 diseases. The American Thoracic Society and European Respiratory Society (ATS/ERS) expert meeting have proposed to classify DILDs into 4 categories, ① known cases, ② granulomatous diseases, ③ idiopathic interstitial pneumonias (IIPs), and ④ other forms. Among them, IIPs also comprise several different clinicopathological entities; however, it was previously thought to be a single disease, idiopathic pulmonary fibrosis (IPF), and resulted in a lot of confusion. Therefore, the ATS/ERS consensus classification provided a standardized nomenclature and diagnostic criteria of IIPs. IIPs are now classified into seven clinicopathologic entities; IPF, nonspecific interstitial pneumonia (NSIP), cryptogenic organizing pneumonia (COP), acute interstitial pneumonia (AIP), respiratory bronchiolitis-associated interstitial lung disease (RB-ILD), desquamative interstitial pneumonia (DIP), and lymphoid interstitial pneumonia (LIP). Because of these complexities, it is not easy to make the diagnosis of the patients with DILDs. This review will present a brief overview of DILDs and summary of diagnostic approaches with highlights on several specific items. An important thing to emphasize is the diagnosis of DILDs should be multi-disciplinary and the final diagnosis should be rendered only after reviewing all of the clinical, radiological, and pathological data of the patient. The more detailed description and discussion on some common diseases of DILDs will be followed by different authors.