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© Korean Medical Association This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons. org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Interstitial lung disease (ILD) is a group of diseases, involving the inflammation and fibrosis of the interstitium of the lung. ILD is classified according to whether or not the cause is known. Known causes of ILDs include inhalation of environmental substances, drugs, infection, and related connective tissue disease. ILD of unknown cause is called idiopathic ILD. The most common form of idiopathic ILD is idiopathic pulmonary fibrosis (IPF). IPF is a chronic progressive fibrosing ILD that results in the decline of lung function with exertional dyspnea, cough, bibasilar inspiratory crackles, and digital clubbing. The incidence of IPF increases with age, and is predominant in men. The most characteristic feature of IPF is a usual interstitial pneumonia (UIP) pattern detected on high-resolution computed tomography (HRCT). The typical HRCT pattern in case of UIP is honeycombing, with or without traction bronchiectasis or bronchiolectasis; this may be superimposed with fine reticulation. The typical distribution of UIP is subpleural, and there is basal predominance with heterogeneity. A definitive diagnosis of IPF in patients with clinically suspected IPF is made when there is presence of a UIP pattern on HRCT. Bronchoalveolar lavage or surgical lung biopsy is not recommended if a UIP pattern is detected on HRCT. However, bronchoalveolar lavage and surgical lung biopsy are required if probable UIP pattern, indeterminate UIP pattern, or an alternative diagnosis pattern are found on HRCT in order to diagnose IPF. A specific combination of HRCT patterns and histopathological patterns requiring multidisciplinary discussion is necessary to rule in IPF or rule it out.