Factors affecting the dimension changes of wood resulting from moisture content changes are discussed. A simple relationship is shown to exist between the shrinkage and the density of small specimens of wood dried under practically stress-free conditions, which indicates that the volumetric shrinkage is practically equal to the volume of water lost below the fiber-saturation point and that the change of fiber cavity dimensions is small. Deviations from this relationship caused by drying stresses are explained. Data for the swelling of wood in aqueous electrolyte solutions and in dry organic liquids are presented and considered from the standpoint of swelling theories. Data on the replacement of water in swollen wood by other liquids are presented.