Currently, more than 200 types of interstitial lung disease (diffuse parenchymal lung disease) are recognized. Due to the incomplete understanding of the pathogenesis of the disease, no system of classification can be regarded to be perfect. This chapter attempts to systematically approach the diagnosis and labelling of interstitial lung diseases, and isolate the possible cause based on the clinical manifestations of disease, associated medical comorbidities, and exposures to potential causative agents. Approaches based on imaging and histopathologic patterns have been considered separately in the chapters that follow.