The antibody repertoire changes with age. This change reflects, in part, the age-associated impairment in the production of a diverse population of naive B cells in the bone marrow and, in part, by the decreased diversification of B cells in the germinal center where affinity maturation and isotype switching takes place. B cell number is strictly regulated and despite the decreased output of B cells by the bone marrow does not decline during aging. Self-renewal of peripheral B cells is sufficient to assure the stability of peripheral B cell number. However, when B cell production is stressed as, for example, following drug-induced lymphopenia, the rate of recovery of B cell number as well as of B cell diversity is compromised in old compared to young mice. Finally, aging is associated with the appearance of B cell clonal expansions which not only limit the diversity of the B cell repertoire but very likely give rise to monoclonal serum immunoglobulins and B cell neoplasms.