Understanding interfacial phenomena has been of direct relevance and practical benefit to extending the use of dental adhesives. Both surface physics, which describes properties of the inorganic materials' interfacial zones from their actual phase boundaries toward the bulk phases of the solids, and surface chemistry, which describes phenomena at the solid/biological interface and beyond it into the variable organic environment, have been important. High-energy materials include solids that are very hard, have high melting points, strong intermolecular forces, and basically crystalline structures, such as dental enamel. Low-energy materials, such as dentinal collagen, salivary films, and the organic resins of restorative materials, are softer, lower melting, and have weaker intermolecular forces, poorer crystallinity, and surface energies generally less than 100 ergs/cm. It has been a properly renewed emphasis on wetting of dental surfaces and their modification by primer coats, displacing or mixing with water and adsorbed proteinaceous films, that has promoted the success of many recently developed fourth-generation dentin adhesives. Their improved wettability for biological phases correlates directly with their better infiltration and anchoring of composites.