Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.