Initially dismissed as artifacts during sample preparation or as cell debris in bacterial cultures, outer membrane vesicles (OMVs) are now widely accepted as facsimiles of the outer membrane naturally secreted by Gram-negative bacteria. Within the last decades, several studies focused on OMV biogenesis resulting in different, in part complementary models to explain OMV release. Notably, vesicle formation seems to be an essential process as neither a bacterial species nor a mutant lacking vesicle release has been reported so far. Based on the complex physiological roles discussed for OMVs, it is likely that parallel strategies for their production have evolved to ensure their release in diverse conditions. Although, we are still far from a comprehensive mechanistic understanding of OMV release, several studies shed some insights in the processes driving the liberation of OMVs from the bacterial surface. Within this chapter, we will discuss the observations resulting in the current models for OMV formation including their regulation and relevance for bacterial physiology.