Proper activation of nuclear factor (NF)–κB transcription factors is critical in regulating fundamental biological processes such as cell survival and proliferation, as well as in inflammatory and immune responses. Recently, the NF-κB signaling pathways have been categorized into the canonical pathway, which results in the nuclear translocation of NF-κB complexes containing p50, and the noncanonical pathway, which involves the induced processing of p100 to p52 and the formation of NF-κB complexes containing p52 (Bonizzi, G., and M. Karin. 2004. Trends Immunol. 25:280–288). We demonstrate that loss of tumor necrosis factor (TNF) receptor–associated factor 3 (TRAF3) results in constitutive noncanonical NF-κB activity. Importantly, TRAF3^−/− B cells show ligand-independent up-regulation of intracellular adhesion molecule 1 and protection from spontaneous apoptosis during in vitro culture. In addition, we demonstrate that loss of TRAF3 results in profound accumulation of NF-κB–inducing kinase in TRAF3^−/− cells. Finally, we show that the early postnatal lethality observed in TRAF3-deficient mice is rescued by compound loss of the noncanonical NF-κB p100 gene. Thus, these genetic data clearly demonstrate that TRAF3 is a critical negative modulator of the noncanonical NF-κB pathway and that constitutive activation of the noncanonical NF-κB pathway causes the lethal phenotype of TRAF3-deficient mice.