All the polymeric composite restorative resins shrink during polymerisation resulting in the development of tensile and/or shear stresses at the tooth/restoration interface. The major part of the contraction stresses develops within 15 minutes after the initiation of polymerisation but with the light activated resins it occurs within seconds after irradiation. The tensile stresses may disrupt the adhesive bonding of the restorative system to the cavity walls resulting in microleakage at the tooth/restoration interface. The properties of the restorative resins which include polymerisation shrinkage during hardening, differences in the coefficients of thermal expansion of the tooth and the restoration, and water sorption of the restoration on exposure to the oral environment, play an important role in determining the marginal gap dimensions and hence microleakage. Microleakage at the enamel/restoration interface has been eliminated by the acid etch technique provided that adequate enamel thickness is present. Microleakage at the dentine/restoration interface, however, is much more difficult to eliminate. None of the dentinal bonding restorative systems eliminates microleakage at the gingival margins of restorations that extend to or beyond the cementum/enamel junction. Microleakage is reduced by using an incremental restorative technique but is increased when the restored teeth are subjected to masticatory stress or occlusal loading. None of the dentinal bonding systems prevents the development of marginal gaps at the dentine/restoration interface when evaluated 10 minutes after placement of the restorations but hygroscopic expansion resulting from water or saline immersion results in a significant reduction in marginal gap dimensions. An increase in the cavosurface margins reduces marginal gap dimensions but it is not dependent on cavity depth.(ABSTRACT TRUNCATED AT 250 WORDS)