The religious life of post-war Britain has become more varied, although Christianity in different forms remains the most influential religion. Yet, the influence of Christianity over British institutions has declined greatly over the last century and a half, although both England and Scotland still retain Established Churches (national churches that have formal links with the monarch and are recognized as the state church). In the post-war period religions other than Christianity, as well as other expressions of Christianity, have made their presence felt. Religious belief and behavior have become more varied and experimental, with the result that the hold of any one religion has weakened. The range of British “religious activity” has increased and its boundaries have become less distinct. These considerations bring us back again to the issue of what we mean by “religion.” This question also bears on judgements about the overall place of religion within society. Until we are able to reach a broadly agreed understanding of what constitutes religion, determining the extent of the influence of religion in society will remain hedged with problems. Yet, debates in Britain about controversial matters such as blasphemy laws, religious broadcasting, the religious education of children and Sunday trading have all been peppered with assertions about the extent to which the population is religious and therefore wishes to protect the place of religion in national life. In these debates, everyday and popular understandings of what is meant by religion were very much to the fore. Many have argued that the hold of religion over society is in decline as a result of secularization—a historical process through which religious beliefs and institutions lose their social significance. To chart the course of such a process and to test claims that religion is facing an irreversible decline, however, again requires that we begin from an agreed understanding of what religion is, the forms it may take, and crucially the extent to which it may change while remaining “religion.” Without this, how can we determine whether it has declined in importance?