Display omitted •Monomers are made inside of cells and exported to the wall by unknown mechanism(s).•Possible mechanisms include transporters such as ATP-binding cassette transporters.•Diffusion of monomers through membranes, driven by polymer formation, is possible.•In gymnosperms, monolignol glucosides represent a possible alternate route to lignin.•Cells can export lignin monomers into the shared walls of neighboring cells. Lignin is a highly abundant polymer in plant cell walls that is essential for land plants’ ability to stand upright and transport water. Inside plant cells, lignin monomers, called monolignols, are made from phenylalanine via a multistep pathway. In the cell wall, monomers move freely, until they encounter stationary oxidative enzymes that determine where the lignin polymer forms. However, it remains unclear how lignin monomers are trafficked from inside the cell to the cell wall. Although multiple lines of circumstantial evidence implicate transporters, additional possible mechanisms include the diffusion of monomers across lipid bilayers and the release of monolignol glucosides stored in vacuoles. There are therefore potentially diverse and overlapping mechanisms of monolignol export.