This article deals with the ontogeny, morphology, and function of bronchus-associated lymphoid tissue (BALT). The surface epithelium of the respiratory tract, including the lymphoepithelium of BALT, is described, with emphasis on infiltrated nonepithelial cells (lymphocytes, macrophages). Special attention is given to antigen handling by the lymphoepithelium and the interaction with BALT and lung macrophages. The structure and function of the diverse types of BALT nonlymphoid cells are discussed. The local immune responses (cellular and humoral reactions), the relationships between local and systemic immune reactions, and the role of BALT in the common mucosal immune system are reviewed. The BALT is compared with Peyer's patches in terms of antigen processing, local immune responses, cell populations, and migration of lymphoid and nonlymphoid cells.