MreB proteins are bacterial homologues of actin that directly determine cell shape and are involved in a range of other cellular processes in non-spherical bacteria. Like F-actin in eukaryotes, MreBs self-assemble into dynamic filamentous structures that are essential for cell viability. Recent studies have demonstrated that the MreB cytoskeletal scaffold governs shape determination by controlling functions related to the bacterial cell wall (probably by recruiting and directing peptidoglycan-synthesizing and modifying proteins). Here I consider general implications for bacterial morphogenesis, and the basis for differences in wall expansion and cylindrical cell shape, based on recent studies aimed to determine the role of MreBs in bacteria with different modes of growth.