Diffuse parenchymal lung diseases can have various clinical and radiological presentations. The high-resolution CT scan has a central role in the diagnostic process of an interstitial disease. As a first step in the analysis of such an exam, one has to look for the major radiological sign and then to describe it to build a differential diagnosis in order to guide the management. The goal of this article is to illustrate this approach with examples of diffuse parenchymal lung diseases.