The effectiveness of fluoridation has been documented by observational and interventional studies for over 50 years. Data are available from 113 studies in 23 countries. The modal reduction in DMFT values for primary teeth was 40-49% and 50-59% for permanent teeth. The pattern of caries now occurring in fluoride and low-fluoride areas in 15- to 16-year-old children illustrates the impact of water fluoridation on first and second molars. The secular changes in caries in Hartlepool, a natural fluoride area in the North East of England, shows only a modest change between 1949 and 1989. Information on the pre-eruptive effects of water fluoridation has been reviewed, suggesting that fluoridation should start at birth to provide optimal protection to primary teeth. At 15 years of age, the maximum DMFS reduction in a fluoridated area was due about half to the pre-eruptive and about half to the post-eruptive effect of fluoride.