The diversified NF-κB transcription factor family has been extensively characterized in organisms from flies to humans. However, homologs of NF-κB and many upstream signaling components have recently been characterized in basal phyla, including Cnidaria (sea anemones, corals, hydras, jellyfish), Porifera (sponges), and single-celled protists including Capsaspora owczarzaki and some choanoflagellates. Herein, we review what is known about basal NF-κBs and how that knowledge informs on the evolution and conservation of key sequences and domains in NF-κB, as well as the regulation of NF-κB activity. The structures and DNA-binding activities of basal NF-κB proteins resemble mammalian NF-κB p100 proteins, and their post-translational activation appears to have aspects of both canonical and non-canonical pathways in mammals. Several studies suggest that the single NF-κB proteins found in some basal organisms have dual roles in development and immunity. Further research on NF-κB in invertebrates will reveal information about the evolutionary roots of this major signaling pathway, shed light on the origins of regulated innate immunity, and may have relevance to our understanding of the responses of ecologically important organisms to changing environmental conditions and emerging pathogen-based diseases. [ABSTRACT FROM AUTHOR]