Lignin is an aromatic heteropolymer that is deposited in cell walls of secondarily-thickened cells. It provides rigidity to cell walls, enabling plants to withstand gravity, and makes water-conducting cells impermeable. For certain industrial applications, such as pulp and paper production or biorefining of lignocellulose into fermentable sugars, lignin is a limiting factor. Lignin has been known for decades to consist of p-coumaryl, coniferyl and sinapyl alcohol, and recent research has revealed a range of other monomers that can be incorporated into the polymer as well. This has allowed engineering lignin amount and composition, resulting in large increases in biomass processing efficiency. We also engineered plants with completely new lignin structures by expressing exotic genes, responsible for the biosynthesis of lignin monomer-like compounds, in lignifying cells. Poplar trees with altered lignin and improved biomass processing have been planted in experimental field trials, to investigate whether the improved properties observed in greenhouse conditions are maintained when trees are grown outdoors.