The occurrence of TDIs can be described as prevalence or incidence. The difference is that prevalence refers to all cases, new or old, in a population at a given time, whereas incidence refers to the number of new patients with a TDI during a given period, generally 1 year, in a specified population. The prevalence rate is therefore higher than the incidence rate. For example, by compar- ing the results from Kaste et al. in the USA in 1996 (11), the prevalence was 18.4% in the age interval 6–20 years, whereas the incidence in Sweden in the age interval 6–19 years presented by Glendor et al. (12) the same year was 12.1 per 1000 individuals. A comparison of the two results in almost the same age interval is therefore 18.4% and 1.21%, respectively. Consequently, 18.4% of the individuals in the age interval 6–20 years in the USA had sustained at least one TDI to their permanent teeth during their lifetime, whereas 1.21% of the individuals in the age interval 6–19 years in Sweden had suffered from at least one TDI during the year of investigation. In other words, incidence conveys information about the risk of contracting a TDI, whereas prevalence tells us how widespread TDIs are.