This article analyzes the best available evidence from the major British social surveys to describe and explain the continuous decline of religion throughout the 20th century. This decline is overwhelmingly generational in nature rather than a product of particular periods such as World War II or the 1960s. Measures of religious affiliation, regular attendance at worship, and religious belief show nearly identical rates of intergenerational decline. Decline has not been offset by any positive age effects in an aging society: Britons do not get more religious as they get older. The intergenerational decline follows clear patterns of transmission of parental religious characteristics to children. Two potential modulators of decline are identified and investigated: immigration of people who are more religious than the existing population and higher fertility rates among the religiously active population. Of these only the former appears of importance. The nonwhite ethnic minority immigrant population is far more religious than the white population; however, the rates of intergenerational decline (between immigrant parents and native-born children) are almost as high as for the white population, leading to an intergenerational convergence of levels of religiosity. Although ethnic minority populations tend to be more religious and have higher fertility rates, there is no differential fertility by religiosity among the population as a whole.