Technical Abstract: Lignin, a phenolic polymer in the secondary-thickened plant cell wall, is the major cause of lignocellulosic biomass recalcitrance toward efficient industrial processing. From an applications perspective, it is desirable that secondary cell walls of second generation bioenergy crops have lignin that is readily degraded by chemical pretreatments. On the other hand, lignin must fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of part of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation in the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites meeting these requirements are made by plants, several of which have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of phenolic compounds by phenolic profiling is discussed, as it serves in pathway elucidation and detection of incorporation of alternative lignin monomers.