Initially identified as a constitutive nuclear factor of kappa light chain immunoglobulin in B lymphocytes (NF-κB), the NF-κB transcription factor family now consists of 5 mammalian members (p50/NF-KB1, p52/NF-KB2, p65/RelA, c-Rel, and Re1B). Individual knockouts of each NF-κB subunit in mice have shown that NF-κB is functionally expressed in nearly every tissue and cell type. The best documented roles for NF-κB lie in their ability to modulate the development, activation, and effector functions of immune cells. NF-κB participates in both innate and adaptive immunity through regulation of target genes including anti-apoptotic molecules, cell cycle regulators, cytokines, surface receptors, and various other immune modulators. This chapter will focus on the contribution of each NF-κB member to immune cell differentiation and effector function, particularly with regard to macrophages, dendritic cells, T lymphocytes, and B lymphocytes. [ABSTRACT FROM AUTHOR]