The purpose of this study was to develop methods to evaluate treatment needs (TNEED) and previous treatment (TDONE) in different populations. In the collaborative study, 'Children's Dental Health in Europe', the methods were used in connection with a clinical examination to document and compare treatment needs in 200 5-year-olds and 200 12-year-olds from each of the 8 participating countries. The study comprised a total of 3200 children from Belgium, Germany, Greece, Ireland, Italy, Scotland, Spain, and Sweden. The method for calculating TNEED was validated by estimation of required time to meet the treatment needs (TNTIME). The calculated TNEED shows a clear relationship with the evaluated TNTIME, and the method is applicable to different population subgroups. The results revealed large differences in both TNEED and TDONE among the different countries and between the two age groups.