An analysis of available data on age distribution from the 1978 census of Tanzania is attempted in order to indicate some of the problems faced by development planners. The author first notes that data by single years of age are not yet available; the analysis presented here is therefore dependent on data for broad age groups, which have been published for villages, wards, and regions. The available data are included in an appendix to the report. Consideration is given to the impact of internal migration on the age and sex distribution of the population, particularly in rural areas.