The lack of adaptation between restorative materials and tooth structure has for many years been a major problem in Dentistry. Studies during this time have emphasized that the margins of restorations are not fixed, inert and impenetrable borders, but dynamic microcrevices which contain a busy traffic of ions and molecules (Going, 1972). The microleakage which results, has been defined as “the clinically undetectable passage of bacteria, fluids and molecules or ions between a cavity wall and the restorative materials applied to it” (Kidd, 1976, b). Microleakage has been implicated in a variety of clinical conditions, including recurrent caries, tooth discolouration, hyper-sensitivity, pulp pathology and hastened breakdown of restorative materials (Kidd, 1976, a and b; Torney et al, 1977). Buonocore (1955) was probably the first to bond resins to tooth structure, etched by the application of phosphoric acid to enamel surfaces. It was found that etching the enamel converted a previously hydrophobic, low energy surface to a more wettable high energy state possessing an increased surface area with numerous retentive sites for resin tag formation. Over the years, the acid-etch technique has been used in a number of dental procedures including fissure sealing, the repair of fractured incisal edges, the repair of hypoplastic and abraded areas, the placement of orthodontic brackets and the splinting of teeth. In recent years, studies have evaluated the ability of the acid-etch technique to improve both the adaptation and retention of composite resins to tooth structure and the marginal seal around composite restorations. However, much of the research examining adaptation of composite resins to tooth enamel has been limited to the study of the enamel tooth surface rather than the enamel of the cavity wall. In addition, previous studies have paid little attention to the effects, on the microleakage, of different etching times and different sizes of the inorganic filler particles and to the influence of aging the restorations on the microleakage. The review of the literature also indicated some controversy concerning the use and effectiveness of low viscosity resins. Since the early 1970’s acid-etching of the enamel has been used increasingly in the placement of composite resin restorations, to provide retention and to minimize marginal microleakage. It was the purpose of this investigation to examine the etched enamel cavity wall, to compare the ability of composite resins, of different particle size and viscosity, to adapt to this cavity wall, and to study the influence, in vitro, of these and other factors, on the microleakage occurring at the margins of restoration.