The aim of this study was to compare third molar development in a London population of self-assigned Black British or other Black ethnicity (the Black British group) with that of self-assigned White British subjects. The significance of population differences in dental age estimation (DAE) in these groups has been debated but not previously studied in the UK. This thesis reviews the literature associated with the maturation of children and young adults, the techniques and challenges associated with age estimation and establishing the 18-year-old threshold, and the evidence for and significance of ethnic variability in dental development. Data was collected from dental panoramic tomographs (DPTs) of 5,590 subjects aged 6.00 -23.99 years: 3,555 White British and 2,035 Black British, aiming for 50 male and 50 female subjects in each 6-monthly age band. At every Demirjian stage (TDS) A-H of all third molars, subjects of Black ancestry were younger compared to those of White ancestry with mean ages for males and females generally at least one year and 1.5 years apart respectively. For the lower left third molar the mean ages at TDS A-H, in both males and females, were highly significantly different (p<0.001). Wide age ranges were seen for all third molar TDS in both ethnic groups. In 17-year-old males, 75% of the Black British group and 43% of the White British group had lower left third molars at TDS G or H. In 18-year-old males, these figures were 88% and 61% respectively. Hypodontia, with or without third molar agenesis (TMA), was approximately twice as likely in the White British group compared to the Black British group, and TMA only was approximately three times as likely. Developmentally missing teeth were shown to be associated with delayed third molar development. These findings confirm the variability of third molar development, the limitations of DAE for determining the 18-year-old threshold, and the important significance of ethnicity in DAE.