Summary: In primary lymphoid organs, such as thymus and bone marrow, B and T lymphocytes differentiate from lymphoid stem cells into mature albeit naïve effector cells. In contrast, secondary lymphoid organs, such as the spleen, lymph nodes, and Peyer's patches (PPs), provide an environment that enable lymphocytes to interact with each other, with accessory cells, and with antigens, resulting in the initiation of antigen-specific primary immune responses. Recently, the analysis of gene-knockout mice has shed light on the signaling pathways, cellular requirements, and molecular mechanisms involved in secondary lymphoid organ development. In particular, signals that converge on the nuclear factor-κB (NF-κB) pathway have been demonstrated to play an important role in both early developmental steps as well as maintenance of secondary lymphoid organ structures. Analysis of the histopathological changes in secondary lymphoid tissues of mice lacking individual Rel/NF-κB family members, upstream kinases, and receptors strongly indicates that activation of the recently described alternative NF-κB pathway by membrane-bound lymphotoxin, via p52–RelB heterodimers, plays a major role during initiation steps of secondary lymphoid organ development. Induction of the classical p50–RelA NF-κB activity, as exemplified by tumor necrosis factor receptor signaling, clearly also contributes, but seems to be involved primarily in later developmental step, such as the proper cellular and structural organization of B-cell follicles.