'Prevalence' refers to the proportion of an age group convicted at some time in their lives. This should be distinguished from 'incidence,' which refers to the proportion of an age group convicted in 1 year. The annual Home Office statistical reports do not provide information about the prevalence of convictions. Perhaps the best method of estimating the prevalence of convictions is to follow up a sample in a longitudinal survey. The only national (British) estimate of prevalence obtained in this way was reported by Wadsworth (1979). This report showed that for males born in England, Scotland, and Wales in 1946, the prevalence of convictions or cautions for indictable offenses was 12.9 percent for males up to the 21st birthday and for females, the figure was 2.03 percent. Several other approaches to obtaining prevalence rates are discussed. The paper observes that ideally what is needed to estimate prevalence is the percentage of those first convicted at each age. However, it is only possible to estimate the percentage of those convicted in each age range. Such an estimate shows that 11.7 percent of males were convicted up to the 17th birthday, 21.76 percent were convicted up to the 21st birthday, and 43.57 percent were convicted up to the 21st birthday, and 43.57 percent were convicted at some time in their lives. For females, the corresponding figures are 2.1, 4.66, and 14.7 percent. Thus, it does not seem unreasonable to predict that, if the Home Office would publish figures to enable a similar analysis to be carried out in 1989, the lifetime prevalence estimate for males would then exceed 50 percent. A table, footnotes, and eight references are given.