One of the more challenging areas of pulmonary medicine remains elucidating the pathogenesis of idiopathic pulmonary fibrosis (IPF). IPF is a devastating disease process with death occurring in the majority of patients within 5 years of diagnosis (1). A major reason for the difficulties in identifying pathogenic mechanisms has been related to the evolution in defining what IPF is. The diagnosis of IPF has been a moving target, and we are still without a gold standard. The generally accepted gold standard is a light microscopy viewing of a surgical lung biopsy revealing a pattern that has been termed “usual interstitial pneumonia”(UIP). To be “confident” of this pattern, it is usually expected that the pathologist be familiar with the range of patterns of acute and chronic lung injury originally described by Dr. Averill Liebow of Yale University, who is credited with the light microscopy recognition of this pattern on surgical lung biopsy or autopsy (1), and that is shown in Figure 1. It was termed “usual” because, of the interstitial pneumonias of unknown cause that he and his colleagues encountered, it was the most common and hence, usual form. The purpose of this historical perspective is to trace the evolution in the concepts of pathogenesis of IPF/UIP.