B cell memory has long been considered the attribute of the sole IgG-positive B cell subset. Since a few years, and due to new B-cell subset identification procedures, increasing heterogeneity has been identified among the memory B cell pool. IgM-positive cells and germinal center-independent subsets are recent additions to the field. This review describes the diversity of memory B cells, as well as controversial issues on their relative contribution to the recall response. The impact of a protracted germinal center response to the specific mobilization of IgM memory B cells is proposed.