Nuclear factor-kappa B (NF-kappa B) is a ubiquitous transcription factor that, by regulating the expression of multiple inflammatory and immune genes, plays a critical role in host defence and in chronic inflammatory diseases. It is a heterodimer, present in the cytoplasm in an inactive form complexed to an inhibitory protein, I kappa B. Many extracellular stimuli, including viruses, oxidants, inflammatory cytokines and immune stimuli, activate NF-kappa B. Once activated, it binds to recognition elements in the promoter regions of inflammatory and immune genes, such as proinflammatory cytokines, chemokines, inflammatory enzymes and adhesion molecules. Glucocorticoids inhibit activated NF-kappa B and this is likely to be important in the anti-inflammatory action of steroids. Novel inhibitors of NF-kappa B are now under development for treatment of inflammatory diseases such as asthma, rheumatoid arthritis and inflammatory bowel disease.