T follicular helper (T_FH) cells are a specialized subset of effector T cells that provide help to and thereby select high-affinity B cells in germinal centers (GCs). To examine the dynamic behavior of T_FH cells in GCs in mice, we used two-photon microscopy in combination with a photoactivatable fluorescent reporter. Unlike GC B cells, which are clonally restricted, T_FH cells distributed among all GCs in lymph nodes and continually emigrated into the follicle and neighboring GCs. Moreover, newly activated T_FH cells invaded preexisting GCs, where they contributed to B cell selection and plasmablast differentiation. Our data suggest that the dynamic exchange of T_FH cells between GCs ensures maximal diversification of T cell help and that their ability to enter ongoing GCs accommodates antigenic variation during the immune response.