Experiments on bonding of acrylic resins to enamel and dentine began in the early 1950s in England with Dr. Oskar Hagger. He developed a monomer based on glycerophosphoric acid dimethacrylate that was chemically cured with sulphinic acid [1]. This was shown in a Swiss patent (no. 211116, 1951) to bond to tooth structure. His work led to the development of Sevitron, an early commercial adhesive [2,3]. In the U.S., Dr. Michael Buonocore made the second, and more important, advance in adhesive dentistry, by demonstrating that acid etching of enamel led to improved resin-enamel bonds using Sevitron-like resin formulations [4]. His rationale for acid-etching enamel was that little adhesion was obtained on unetched enamel, which he correctly surmised lacked microscopic porosities for resin infiltration. He knew that concentrated (85 wt%) phosphoric acid was used in industry to pre-treat metal surfaces prior to painting or resin coating; thus, it was logical for him to use 85% phosphoric acid for 30 s to etch enamel, followed by water rinsing. The results of his work were very controversial at the time. Many researchers regarded Dr. Buonocore's approach as unconventional and reckless because he advocated the use of dangerous, industrial-strength acids in the oral cavity. Over the next 10 years, many investigators confirmed the utility of acidetching enamel to increase resin-enamel bond strengths. The concentration of the phosphoric acid was subsequently reduced to 50% [5], and more recently to 32-37%. With the recognition that primary tooth enamel surfaces were largely aprismatic, etching times of 120 s were commonly used for bonding procedures for primary teeth [6]. Those etching times have been reduced to 60 s [7] and, more recently, 20-30s [8-10] for aprismatic enamel for bonding of pit and fissure sealants and orthodontic brackets.Phosphoric acid etching worked so well for retention of pit and fissure sealants that it was natural to adopt the same acid on bur-cut enamel cavosurface margins [11]. Both a reduction in the acid concentration as well as etching time [12][13][14] had been proposed. Despite the availability of alternative enamel etchants such as pyruvic, citric, oxalic, nitric or maleic acid, phosphoric acid still remains the etchant of choice, with the contemporary adoption of a reduced etching time to 15 s for both prismatic and aprismatic enamel. The solutions used to etch enamel were also made into gels to permit better control of these acids, since acid etching of dentine was erroneously thought to devitalize pulps [15]. For bonding to cut enamel, it was further observed that even a 5-s etching time [16,17], or a phos-