NF-κB was first discovered and characterized 25 years ago as a key regulator of inducible gene expression in the immune system. Thus, it is not surprising that the clearest biological role of NF-κB is in the development and function of the immune system. Both innate and adaptive immune responses as well as the development and maintenance of the cells and tissues that comprise the immune system are, at multiple steps, under the control of the NF-κB family of transcription factors. Although this is a well-studied area of NF-κB research, new and significant findings continue to accumulate. This review will focus on these areas of recent progress while also providing a broad overview of the roles of NF-κB in mammalian immunobiology.