Abstract: Dental adhesives are used to bond composite resins to tooth structure. The earliest bonding systems required an acid-etch technique and were only compatible with enamel. The challenge has always been to predictably bond to enamel and dentin simultaneously. The material should have such a property that it should have better retention to the dentin and it should be able to withstand forces of occlusion and mastication. The ideal requirements of an effective dentin adhesive system include the ability to thoroughly infiltrate the collagen pathways and partially demineralized zone, to encapsulate the collagen and hydroxyapatite crystallites, to produce a well polymerized durable hybrid layer with high bond strengths. As a hybrid layer is created by a mixture of the dentin organic matrix, residual hydroxyapatite crystallites, resin monomers and solvents, aging may affect each of the individual components differently or may result in synergistic combinations of degradation phenomena within the hybrid layer.