The structural integrity of cell walls in wood is maintained by matrix polysaccharides through chemical interactions among themselves and with both cellulose and lignin. During wood formation, matrix polysaccharides are deposited in the cell walls and subsequently modified by spontaneous and/or enzyme-mediated processes. These processes are highly regulated during successive stages of wood formation, including cell division, expansion and secondary (or further tertiary) cell wall layer deposition. They are also adjusted in response to various endogenous and environmental signals by developing xylem cells, with corresponding modifications of cell wall properties. In addition, there is substantial variation among taxonomic groups of plants in the chemical composition of matrix wood polysaccharides. This chapter discusses these dynamic changes in matrix polymers during wood development and adaptive responses to environmental stresses, the matrix variation among cell types and between hardwoods and softwoods and the functional significance of this dynamic variation for wood cell wall properties.