The outer membrane (OM) of Gram-negative bacteria functions as a protective barrier. It is unusual because the OM bilayer is asymmetric; the inner leaflet is composed of phospholipids, but the outer leaflet is made of lipopolysaccharide (LPS). Two kinds of proteins are found in the OM. Lipoproteins are inserted into the inner leaflet of the OM by posttranslationally attached lipid moieties. Integral OM proteins are β-barrel proteins (OMPs). We have used genetic, bioinformatic and biochemical strategies to identify and characterize the cellular components required for LPS and OMP biogenesis. After describing what is known about these essential cellular processes, I will focus on the genetic approaches we are taking to probe the mechanism by which β-barrel assembly machine folds and inserts OMPs into the OM. This machine is composed of a large β-barrel protein, BamA, and four lipoproteins, Bam BCDE. My talk will focus on how the two essential proteins, BamA and BamD, communicate with each other, and how this communication controls the conformation and activities of each.