The diffuse interstitial lung diseases (ILD) are a very complex group of disorders, of which more than 200 specific disease entities are known. All ILD are characterized by a homogenous or heterogenous affection of the lung parenchyma. In recent years there was a renewed interest in the ILD because new concepts have been developed on aetiologic influences, on pathogenetic mechanisms (of inflammation versus active fibrosis) and on genetic susceptibility and consequently on the diagnostic-therapeutic management. Yet, there remain many questions still, e.g. concerning the exact incidence and prevalence of the majority of these diseases and concerning the genetic aspects. More specifically much research has been carried out on the important group of "idiopathic interstitial pneumonias" (IIP) and in particular on "idiopathic pulmonary fibrosis" (IPF). Recently several international consensus reports have been published with guidelines on the histologic and clinical definitions and on the classification and the diagnostic-therapeutic management.