Lymphocytic interstitial pneumonia (LIP) is a clinicopathologic term that relates histologically to a dense interstitial infiltrate of mainly T cells, plasma cells, and histiocytes, with germinal centers often identified. Its precise etiology is unknown, but there are strong clinical associations with several autoimmune disorders, as well as both congenital and acquired immunodeficiency syndromes. It may overlap histologically with both extrinsic allergic alveolitis and nonspecific interstitial pneumonia, and therefore close clinical/radiological association is essential for diagnosis. LIP also overlaps clinically and histologically with follicular bronchitis/bronchiolitis, the latter showing reactive lymphoid hyperplasia with a peribronchiolar distribution predominantly comprising lymphoid follicles. LIP may also be histologically indistinguishable from nodular lymphoid hyperplasia and lymphomas arising from mucosa-associated lymphoid tissue (MALT) but can usually be differentiated via analysis of clinical and imaging data plus assessment of immunohistochemistry and gene rearrangement studies. Other entities include lymphomatoid granulomatosis, intravascular lymphomatosis, Castleman's disease, primary pleural lymphomas, primary effusion lymphomas, plasmacytomas, and secondary involvement by lymphoma, but these should all be readily distinguishable from lymphocytic interstitial pneumonia if all clinical, imaging, and histological data are apparent.