Technical Abstract: Lignins are complex and irregular polymers occurring in the cell walls of vascular plants. An understanding of their nature is evolving as a result of detailed structural investigations aided by recent improvements in analytical methodology and the availability of lignin-biosynthetic-pathway mutants and transgenics. The picture that emerges from these studies is that the lignin polymer is generated by oxidative phenolic coupling reactions under chemical control, where monomer radicals couple endwise with radicals on the polymer chain. The combinatorial linkage synthesis and the random generation of new optical centers each time a monolignol couples at its sidechain Š-position cascade to create polymers with enormous variation in primary structure. Furthermore, lignins may derive in part from monomers and conjugates other than the three primary monolignols (p-coumaryl, coniferyl, and sinapyl alcohols). The plasticity of the combinatorial polymerization reactions may allow monomer substitution when the biosynthesis is perturbed, at times introducing significant variations into the final structure. Lignification is plainly a strategically evolved process allowing plants considerable flexibility in dealing with various environmental stresses. The malleability offers significant opportunities to engineer the structures of lignins beyond the limits explored to date.