Polymerization shrinkage of composite restorative materials causes clinical concerns because it introduces residual stresses in restored teeth. These stresses can, among other things, propagate enamel cracks, bring about microleakage, and cause postoperative sensitivity. The amount of shrinkage stress does not depend only on how much a composite contracts, but also on the elastic modulus ("stiffness") of the composite, the shape of the cavity, the established bonding between the tooth and restoration, etc. The relationships between these many factors can be described by universal physical laws. To analyze shrinkage stresses, theoretical models must be used that relate the various shrinkage properties and clinical conditions via physical laws. Models will assist us to rethink and optimize established paths to clinical success.