A large and diverse group of pathologic conditions manifests clinically and radiologically as diffuse parenchymal lung disease. Diffuse interstitial lung diseases (ILDs) encompass mainly inflammatory processes that involve the structural elements of this organ. Some ILDs are caused by infections, but most are the result of immunologic, environmental, or toxic mechanisms. Currently, less morbid sampling techniques have increased dramatically the probability that pulmonologists will be faced with establishing a specific and clinically relevant diagnosis using surgical lung biopsy material. Most of the concepts presented in this article have been established using this type of specimen. In the early years of surgical lung biopsy, a small number of diffuse inflammatory conditions came to light that exclusively involved the lungs and did not seem to be caused by infection, toxin, sarcoidosis, pneumoconiosis, or neoplasm. In this article, these idiopathic disorders are discussed in the context of their dominant pathologic findings rather than presented as a separate group of entities.