This review describes some of the biochemical and physiological factors that should be considered in designing in vitro models of the oral cavity. The significance of the non-Newtonian properties and internal structures of saliva are reported as well as the ability of saliva to form biological films on all types of solid surfaces. The presence of compositional biochemical variations between absorbed salivary films is discussed for solid surfaces having different surface chemistries. The significance of the presence of small amounts of organic material in enamel and dentin is also stressed. Finally, by selecting other, more easily available secretional products than saliva, more relevant model systems for materials testing in vitro may be established than if only simple aqueous salt solutions are used.