The outer membrane (OM) of Gram-negative bacteria is an essential organelle that prevents the indiscriminate diffusion of small molecules while permitting the selective entry of vital nutrients. To achieve this barrier function, the lipid to protein ratio of the OM must be carefully maintained despite dynamic growth conditions. Severe alterations in envelope composition lead to loss of OM integrity and cell death, a fact that had for decades hampered the search for viable mutations in genes responsible for the expression of OM assembly factors. Moderate changes in composition lead to increased OM permeability, and this general characteristic can result from an array of different mutations. Genetic characterization of hyperpermeable mutants can be complicated because the resulting phenotypes are often subtle and pleiotropic. Recent evidence demonstrates that specific mutations that alter OM permeability become distinguishable through the use of toxic small molecules. This exciting realization allowed the design of powerful selections to probe for genes expressing factors involved in OM biogenesis. Bacteria carrying undefined OM permeability mutations can be used in selections that demand a distinct and reproducible set of genetic solutions that restore OM barrier function and prevent specific toxic compounds from reaching an intracellular target. We review how this discovery led to the identification of YaeT (Omp85), an ancient OMP assembly factor conserved in Gram-negative bacteria and eukaryotes. Moreover, we evaluate the genetic and biochemical interactions between YaeT and other components that also play a role in the translocation and assembly of OMPs in the OM.