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Determining the chemical composition of biological materials is paramount to the study of natural phenomena. Here, we describe the composition of model gram-negative outer membranes, focusing on the predominant assembly, an asymmetrical bilayer of lipid molecules. We also give an overview of lipid biosynthetic pathways and molecular mechanisms that organize this material into the outer membrane bilayer. An emphasis is placed on the potential of these pathways as targets for antibiotic development. We discuss deviations in composition, through bacterial cell surface remodeling, and alternative modalities to the asymmetric lipid bilayer. Outer membrane lipid alterations of current microbiological interest, such as lipid structures found in commensal bacteria, are emphasized. Additionally, outer membrane components could potentially be engineered to develop vaccine platforms. Observations related to composition and assembly of gram-negative outer membranes will continue to generate novel discoveries, broaden biotechnologies, and reveal profound mysteries to compel future research. 255 Click here to view this article's online features: ANNUAL REVIEWS Further A nn u. R ev . M ic ro bi ol . 2 01 6. 70 :2 55 -2 78 . D ow nl oa de d fr om w w w .a nn ua lre vi ew s.o rg A cc es s p ro vi de d by H ar va rd U ni ve rs ity o n 01 /1 0/ 18 . F or p er so na l u se o nl y. MI70CH14-Trent ARI 10 August 2016 15:5 Glycerophospholipid (PL): a membrane lipid with a glycerol-3-phosphate backbone with fatty acyl chains esterified to positions 1 and 2 Lipopolysaccharide (LPS): a glycolipid localized to the outer leaflet of the outer membrane comprising lipid A, coreoligosaccharide, and O-antigen domains