Factors that need to be considered to explain the observed cupping pattern of panel paintings are reviewed. The anisotropic shrinkage and the action of the paint layer, both a reinforcement and a moisture barrier are the most obvious ones, but the progressive contraction of ancient wood is usually invoked. Although the aging process of wood could be responsible for some of it, several mechanical processes may lead to compression set: mechano-sorption, microbuckling, or even the recovery of locked-in growth strains.