B cell activating factor belonging to the tumor necrosis factor family (BAFF) is a tumor necrosis factor (TNF) superfamily member best known for its role in the survival and maturation of B cells. BAFF is a ligand for three TNF receptor superfamily members: B-cell maturation antigen (BCMA), transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), and BAFF receptor (BAFF-R). Among them, BAFF-R plays the central role in the BAFF system, whereas TACI plays the inhibitory role. BAFF/BAFF receptors appear to span nearly all stages of B-lineage differentiation, ranging from the development, selection, and homeostasis of naive primary B cells to the maintenance of long-lived bone marrow plasma cells. Furthermore, excessive BAFF rescues self-reactive B cells from anergy, which may play a crucial role in the induction and development of autoimmunity. Mice overexpressing BAFF exhibit increased B cell numbers in spleen and lymph node and autoimmune phenotype similar to patients with systemic lupus erythematosus (SLE) and Sjögren's syndrome. Furthermore, inhibition of BAFF by TACI-Ig and BAFFR-Ig has been successful in treating murine models of SLE and rheumatoid arthritis. In humans, previous reports have shown elevated serum BAFF levels in SLE, rheumatoid arthritis, Sjögren's syndrome, and systemic sclerosis patients. Thus, the dysregulation of BAFF/BAFF receptor system may contribute to induction and development of autoimmune diseases and become one of important therapeutic targets.