The transcription factor NF-κB plays a major role in coordinating innate and adaptative immunity, cellular proliferation, apoptosis and development. Since the discovery in 1991 that NF-κB may be activated by H₂O₂, several laboratories have put a considerable effort into dissecting the molecular mechanisms underlying this activation. Whereas early studies revealed an atypical mechanism of activation, leading to IκBα Y42 phosphorylation independently of IκB kinase (IKK), recent findings suggest that H₂O₂ activates NF-κB mainly through the classical IKK-dependent pathway. The molecular mechanisms leading to IKK activation are, however, cell-type specific and will be presented here. In this review, we also describe the effect of other ROS (HOCl and ¹O₂) and reactive nitrogen species on NF-κB activation. Finally, we critically review the recent data highlighting the role of ROS in NF-κB activation by proinflammatory cytokines (TNF-α and IL-1β) and lipopolysaccharide (LPS), two major components of innate immunity.