One of the primary physiological roles of nuclear factor-kappa B (NF-κB) is in the immune system. In particular, NF-κB family members control the transcription of cytokines and antimicrobial effectors as well as genes that regulate cellular differentiation, survival and proliferation, thereby regulating various aspects of innate and adaptive immune responses. In addition, NF-κB also contributes to the development and survival of the cells and tissues that carry out immune responses in mammals. This review, therefore, describes the role of the NF-κB pathway in the development and functioning of the immune system.