NF-κB activity in mammalian cells is regulated through the IκB kinase (IKK) complex, consisting of two catalytic subunits (IKKα and IKKβ) and a regulatory subunit (IKKγ). Targeted deletion of Ikkβ results in early embryonic lethality, thus complicating the examination of IKKβ function in adult tissues. Here we describe the role of IKKβ in B lymphocytes made possible by generation of a mouse strain that expresses a conditional Ikkβ allele. We find that the loss of IKKβ results in a dramatic reduction in all peripheral B cell subsets due to associated defects in cell survival. IKKβ-deficient B cells are also impaired in mitogenic responses to LPS, anti-CD40, and anti-IgM, indicating a general defect in the ability to activate the canonical NF-κB signaling pathway. These findings are consistent with a failure to mount effective Ab responses to T cell-dependent and independent Ags. Thus, IKKβ provides a requisite role in B cell activation and maintenance and thus is a key determinant of humoral immunity.