Interstitial lung disease (ILD) is a difficult area for pathologists because of numerous overlapping morphologic patterns, some of which correspond to specific disease entities and some of which can be seen in a variety of different diseases. The clinical and physiologic changes in ILD tend to be nonspecific and are usually not helpful in diagnosis, but high-resolution CT scan (HRCT) images can establish the diagnosis with high accuracy in many cases, often obviating the need for biopsy. Some forms of ILD have a genetic component, and it is now recognized that patients with short telomeres are at high risk of fibrotic forms of ILD. Respiratory clinicians sometimes use percent confidence levels as a way to approach ILD diagnoses, but this approach is not suitable for pathologists; rather, we recommend providing a ranked differential diagnosis. Some proportion of ILD fall into the category of progressive pulmonary fibrosis, that is, disease that progresses without obvious external stimuli. The general approach to treatment of these diseases is discussed.