Discovery and characterization of the tumor necrosis factor (TNF) family member B‐lymphocyte stimulator (BLyS) has opened a novel chapter in the role of TNF family members in the homeostatic control of lymphocyte populations. BLyS and its sister cytokine APRIL (a proliferation‐inducing ligand) act primarily as soluble trimers and serve to regulate the steady‐state numbers of nearly all B‐cell compartments. This homeostatic regulation is accomplished through the regulation of B‐cell production rates, selection thresholds, and lifespan. Differential expression of the three BLyS receptors during differentiation and activation provides related yet distinct homeostatic niches for follicular, marginal zone, and memory B‐cell subsets.