The cell envelope of Gram-negative bacteria is a complex macromolecular structure that is essential for their viability. Little is known on how the proteins which are secreted to the envelope fold into their unique three-dimensional structure. Several folding factors, including chaperones and protein folding catalysts involved in disulfide bond formation, have been identified in the periplasm. The characterization of these proteins has advanced our understanding of envelope biogenesis, although many fundamental questions remain unanswered. In particular, we still do not know how β-barrel proteins are transported through the periplasm and inserted into the outer membrane. Here, we discuss the recent discoveries that have shed new light on the mechanisms that ensure the correct folding of envelope proteins. We have paid particular attention to the significant contribution of proteomic studies.