Idiopathic interstitial pneumonias (IIP), whose pathogenetic mechanisms are still largely unknown, represent a significant portion of the complex spectrum of DPLD. A classification based on histological criteria was first undertaken by Liebow and Carrington in 1969; the current classification was published in 2002 by the American Thoracic Society (ATS) and the European Respiratory Society (ERS). This latter classification is also essentially based on clinical, radiological and histopathological criteria. Overlaps between the histopathological patterns are very much a feature. If the molecular causes for the development of IIP are discovered, a further revision of this classification will be expected. Besides the defined entities of IIP described below, there is also a category called “unclassifiable interstitial pneumonia,” a term that should always be used if a clear classification (e.g. in the case of terminally altered lung tissue) is no longer possible.