Phenolic compounds are known to be released into the soil by root exudates and litter inputs thus alterations in foliar and root chemistry have important implications for plant-litter-soil interactions and ecosystem function. The general pathways of phenol biosynthesis are known, but the regulation and controlling factors determining the quality and quantity of phenols in plant tissue remain active and controversial areas of research. Much of the controversy arises because regulating factors at several levels, ranging from intrinsic species- and genotype-specific factors- to various extrinsic environmental factors, may interact with each other resulting in a wide variation of plant polyphenol production among and within species through time and space. Production of secondary compounds in plants may be induced at the level of the phenotype in response to abiotic factors which might additionally affect genotype selection in the long term. Both short-term phenotypic variations and genotype selection at larger time-scale have implication for litter quality input and soil phenol contents. The multitude of intrinsic and extrinsic factors involved in the regulation of plant phenol production is comprehensively discussed here. In this book the authors focus on specific issues and the causes that affect the biosynthesis of plant phenols and, in turn, the composition and concentration of soil phenolic compounds.