Rel/NF-κB proteins constitute a family of structurally-related transcription factors that are regulated by interaction with a family of regulatory proteins, the IκB proteins. In this review, we will familiarize the reader with the known Rel/NF-κB and IκB molecules, and will discuss the types of interactions that occur between these proteins to regulate their function. In addition, we will describe what is known about the structure of Rel/NF-κB transcription complexes on DNA. Lastly, we will discuss the wide variety of biological stimuli and conditions which activate NF-κB and the genes whose expression are regulated by this family of transcription factors.