Mycolic acids (MAs) are the hallmark of the cell envelope of Mycobacterium tuberculosis and related species and genera. MAs are unique and complex molecular structures that are found either unbound, extractable with specific organic solvents such as esters of trehalose or glycerol, or esterifying the terminal pentaarabinofuranosyl units of arabinogalactan (AG), the polysaccharide that forms the insoluble cell wall skeleton together with peptidoglycan. Both forms probably play a crucial role in the extraordinary architecture and impermeability of the cell envelope, taking part in the two leaflets of the mycobacterial outer membrane, also referred to the mycomembrane, recently visualized using electron microscopy method. This concept has been established in Corynebacterium glutamicum, a species belonging to a genus that exhibits diverse features of the mycobacterial cell envelope, remarkably the presence of an MA-containing outer membrane.