T_reg cells are essential for the maintenance of immune homeostasis and prevention of autoimmunity. In humoral immune responses, loss of T_reg cell function causes increased levels of serum autoantibodies, hyper-IgE, spontaneous generation of germinal centres, and enhanced numbers of specialised T follicular helper cells (T_fh cells) controlled by the lineage-defining transcription factor BCL-6 (B-cell lymphoma 6). Recent studies have demonstrated that a subset of T_reg cells [T follicular regulatory (T_freg) cells] are able to co-opt the follicular T-cell program by gaining expression of BCL-6 and travelling to the follicle where they have an important role in the control of expansion of T_fh cells and the germinal centre reaction. However, the mechanisms by which they exert this control are still under investigation. In this review, we discuss the effects of T_reg cells on humoral immunity and the mechanisms by which they exert their regulatory function.