Context.-Diagnosis and classification of lymphomas are based on the morphologic, immunologic, and genetic features that the lesional cells share with their normal B and T lymphocyte counterparts. Primary pulmonary lymphomas account for 0.3% of primary lung neoplasms and less than 0.5% of all lymphomas.Objective.-To describe and summarize the clinical and histopathologic features of the primary pulmonary lymphoma and secondary involvement of the lung by lymphoma.Data Sources.-Peer-reviewed published literature and personal experience.Conclusions.-Diagnosis of clonal lymphoid proliferations in the lung has evolved owing to the greater utility of molecular and flow cytometric analysis of tissue. Further studies are needed to best define the clinical and prognostic features, as well as search for targeted therapy for these patients with rare neoplasms.(Arch Pathol Lab Med. 2013;137:382-391; doi: 10.5858/ arpa.2012-0202-RA) P rimary pulmonary lymphoma is defined as clonal lymphoid proliferation affecting one or both lungs (parenchyma and/or bronchi) in a patient with no previous extrapulmonary involvement at the time of diagnosis or during the subsequent 3 months. Primary lymphoma of the lung is a rare disorder and represents only 0.3% of all primary pulmonary malignancies, less than 1% of all the cases of non-Hodgkin lymphoma, and 3% to 4% of all the extranodal manifestations of non-Hodgkin lymphoma.