The cell envelope of gram-negative bacteria consists of two membranes, the inner and the outer membrane, that are separated by the periplasm. The outer membrane consists of phospholipids, lipopolysaccharides, integral membrane proteins, and lipoproteins. These components are synthesized in the cytoplasm or at the inner leaflet of the inner membrane and have to be transported across the inner membrane and through the periplasm to assemble eventually in the correct membrane. Recent studies in Neisseria meningitidis and Escherichia coli have led to the identification of several machineries implicated in these transport and assembly processes.