The maintenance of mature B cells hinges on signals emitted from the BAFF-R cell-surface receptor, but the nature of these signals is incompletely understood. Inhibition of canonical NF-kappaB transcription factor activity through ablation of the essential scaffold protein NEMO arrests B cell development at the same stage as BAFF-R deficiency. Correspondingly, activation of this pathway by constitutively active IkappaB Kinase2 renders B cell survival independent of BAFF-R:BAFF interactions and prevents proapoptotic PKCdelta nuclear translocation. In addition, canonical NF-kappaB activity mediates differentiation and proper localization of follicular and marginal zone B cells in the absence of BAFF-R, but not CD19. By replacing BAFF-R signals, constitutive canonical NF-kappaB signaling, a hallmark of various B cell lymphomas, causes accumulation of resting B cells and promotes their proliferation and survival upon activation, but does not per se induce lymphomagenesis. Therefore, canonical NF-kappaB activity can substitute for BAFF-R signals in B cell development and pathogenesis.