Acid-related tooth structure loss (erosion) is increasingly becoming the focus of dental medicine, as shown by the German Oral Health Studies (DMS). Manufacturers of dental care products are also launching more and more products on the market that are intended to protect against this loss of tooth structure with special active ingredients. Dental care products containing hydroxyapatite as an active ingredient to combat erosion have been available for some time. So far, however, little is known about the effects, mode of action and mechanism of action of the new active ingredient, particularly in dental care products. The aim of this dissertation was therefore to show whether the new active ingredient hydroxyapatite can be used for the symptomatic therapy of acid-induced tooth structure loss. In addition, the mode of action and the mechanism of action were to be analyzed. For this purpose, the following questions should be answered: • Can hydroxyapatite effectively reduce erosively induced tooth structure loss compared to negative control? • Can hydroxyapatite effectively reduce erosively induced tooth structure loss compared to established active ingredients? • Are precipitates detectable on eroded enamel after hydroxyapatite application? • In what form is hydroxyapatite present in toothpastes and mouth rinses? To answer these questions, the loss of substance of human enamel samples was measured in an erosion/abrasion model after treatment with the test products. Furthermore, eroded enamel surfaces were examined for morphological changes by scanning electron microscopy after in vitro or in situ application of the dental care products. In addition, further analyses of the relevant parameters of the dental care products, such as pH determination and determination of fluoride concentration and calcium concentration, were carried out. The particle content of the toothpastes was determined, the particles of the toothpastes were analyzed for their shape, size and surface texture, and the RDA /REA values were determined. Three hydroxyapatite formulations (two toothpastes, one mouth rinse), two combination toothpastes (containing hydroxyapatite and fluoride), two products containing tin and fluoride (positive control; one toothpaste, one mouth rinse) and one toothpaste containing no active ingredients (negative control) were investigated. In the substance loss measurement, no significant reduction by hydroxyapatite-containing products (hydroxyapatite formulations and combination toothpastes) could be detected compared to the negative control, both under erosion alone and under additional abrasion. In some applications, the loss of substance even increased. Also compared to the positive controls, almost all hydroxyapatite-containing products showed significantly worse results. Only when one of the combination toothpastes and the hydroxyapatite-containing mouth rinse was applied with additional abrasion, no significant differences could be found compared to the application of the positive control toothpaste. The positive control mouth rinse was significantly superior to all other products. No correlation to particle content, pH and RDA and REA values was evident. There seems to be at least a partial correlation to the fluoride content and the concentration of calcium ions values. Morphological structural analysis of the eroded enamel surfaces in vitro showed no precipitation by the products. The partially discernible changes seem to be caused by the remineralization solution used rather than by the products itself. No precipitation could be detected in situ either. Here, the partially detectable changes appear to be caused by pellicle coverage and to be subject-dependent rather than product-dependent. Analysis of the particulate phase and determination of the concentration of calcium ions suggests that the hydroxyapatite is not in a highly crystalline form, but either bound to the particles or in solution in the dental care products containing hydroxyapatite. The results of the dissertation show that hydroxyapatite as an active ingredient has no positive effect compared to the negative control as well as the established active ingredients regarding acid-induced tooth structure loss. Precipitation on eroded tooth surfaces could not be demonstrated. The results also suggest that hydroxyapatite is not highly crystalline but bound on particles or in solution. Consequently, oral care products containing hydroxyapatite should not be used in symptomatic erosion therapy. A deviation from the previous recommendation to use products containing stannous and fluoride does not appear to be reasonable.