Pulmonary lesions encountered by the pathologist in which histiocytes are the dominant finding histologically are reviewed. Lesions discussed include neoplasms of histiocytes and nonneoplastic processes. The nonneoplastic processes are divided into those that present as nodular histiocytic proliferations in the lung, those that present as diffuse proliferations of histiocytes in the lung, and those with a mixed pattern. Entities discussed include pulmonary Langerhans' cell histiocytosis, pneumonoconioses, infections, diffuse panbronchiolitis, crystal storing histiocytosis, respiratory bronchiolitis, alveolar hemorrhage, eosinophilic pneumonia, obstructive pneumonia, exogenous lipoid pneumonia, some drug reactions, and some metabolic/storage diseases. Entities of uncertain histogenesis, including Rosai-Dorfman disease and Erdheim-Chester disease, are also discussed. Qualitative features of the histiocytes are addressed, including the presence of foreign dust, hemosiderin, foamy change, and histiocytes showing features of Langerhans' cells.