NF-${kappa}B$/Rel transcription factor family participates in diverse biological processes including embryo development, hematopoiesis, immune regulation, as well as neuronal functions. In this review, the NF-${kappa}B$/Rel signal transduction pathways and their important roles in the regulation of immune system will be discussed. NF-${kappa}B$/Rel members execute distinct functions in multiple immune cell types via the regulation of target genes essential for cell proliferation, survival, effector functions, cell trafficking and communication, as well as the formation of lymphoid architecture. Consequently, proper activation of NF-${kappa}B$/Rel during immune responses to allergens, auto-antigens, allo-antigens, and pathogenic infection is crucial for the integrity of host innate and adaptive immunity.