Gram-negative bacteria shed extracellular outer membrane vesicles (OMVs) during their normal growth both in vitro and in vivo. OMVs are spherical, bilayered membrane nanostructures that contain many components found within the parent bacterium. Until recently, OMVs were dismissed as a by-product of bacterial growth; however, findings within the past decade have revealed that both pathogenic and commensal bacteria can use OMVs to manipulate the host immune response. In this Review, we describe the mechanisms through which OMVs induce host pathology or immune tolerance, and we discuss the development of OMVs as innovative nanotechnologies.