Many products that claim to have caries-preventive effects are available to the general public. Different and sometimes even contradictory claims of effectiveness are given in advertisements. Dentists would like to give good information and advice to their patients and the public with respect to the efficacy of different products. In medicine and dentistry there are guidelines which consider the ethics, design, conduct as well as the recording and reporting of clinical studies. The effects of clinical trials in subjects with a high risk for caries or in a population with a high caries prevalence are not comparable to the situation in most industrialized countries today. The sample should, therefore, include subjects with a range of prevalences and of risks for a valid assumption of efficacy. Reliable data can be obtained if a strict control of the design and management of the clinical trials is maintained over the duration of the experiment. New diagnostic methods used in the study must be validated and could lead to trials of a shorter duration. For a new fluoride product the bioavailability of the ion should be demonstrated. The results must have the necessary characteristics to be included in a meta-analysis. Thesis: The caries-preventive effect of a fluoride product cannot be accurately measured in populations with a variety of preventive products in use. On the other hand, the measured effect in populations with caries prevalence higher than those of the country where it will be used is not comparable to the real situation.