Some of the changes which occur in wood during the transition from sapwood to heartwood have been reviewed. The nature of these changes suggest that heartwood formation is a regulatory process serving to keep the amount of sapwood at an optimum level. The pattern formed by the transition of cells from sapwood to heartwood suggests that heartwood development is controlled by a centripetally-translocated growth-active substance. The nature of the heartwood transformation is indicative of a developmental process rather than a deterioration of cell function with age so that death of the parenchyma cells is the result and not the cause of heartwood formation.