Once a tree is cut and harvested the wood becomes subject to several environmental factors which affect the degree of deterioration that occurs. These environmental factors are described. The most important environmental factor is the relative humididty (RH). A convenient guideline to remember is that a 10per thousand change in relative humidity can result in a 1per thousand change in the width of the wood and about half that much of a dimensional change in the radial direction. Change in longitudinal direction is negligible. Warp can occur when wood is subjected to changes in relative humidity. Other environmental factors that cause deterioration of wood are also discussed. They include temperature changes, restraints on wood, light and biopredation. This is the fourth in a 10-part series of articles on furniture conservation.