Most bacteria release membrane vesicles (MVs) that contain specific cargo molecules and have diverse functions, including the transport of virulence factors, DNA transfer, interception of bacteriophages, antibiotics and eukaryotic host defence factors, cell detoxification and bacterial communication. MVs not only are abundant in nature but also show great promise for applications in biomedicine and nanotechnology. MVs were first discovered to originate from controlled blebbing of the outer membrane of Gram-negative bacteria and are therefore often called outer-membrane vesicles (OMVs). However, recent work has shown that Gram-positive bacteria can produce MVs, that different types of MVs besides OMVs exist and that, in addition to membrane blebbing, MVs can also be formed by endolysin-triggered cell lysis. In this Review, we provide an overview of the structures and compositions of the various vesicle types and discuss novel formation routes, which may lead to distinct vesicle types that serve particular functions.