The outer membrane (OM) of Gram-negative bacteria is an essential organelle that protects cells from external aggressions and mediates the secretion of virulence factors. Efficient assembly of integral OM β-barrel proteins (OMPs) is crucial for the correct functioning of the OM. Biogenesis of OMPs occurs in a stepwise manner that is finalized by the β-barrel assembly machinery (BAM complex). Some OMPs further require the translocation and assembly module (TAM) for efficient and correct integration into the OM. Both the BAM complex and the TAM contain a protein of the Omp85 superfamily and distinct interacting factors. Their mechanism of action, however, remains largely elusive. We summarize and discuss recent structural and biochemical analyses that are helping to elucidate the molecular pathways of OMP assembly.