The size of the UK Jewish population has always been a source of uncertainty for demographers. Following considerable discussion and testing, a voluntary question on religion was introduced into the 2001 Census, which afforded the first opportunity to provide definitive answers to the socio-demographic make-up of Jews in Britain. However, examination of the 2001 Census figures and data from several large surveys suggests that the census population of 266,740 British Jews by religion is probably a considerable undercount. Jews are increasingly defining themselves in ethnic rather than religious terms, so there is reason to question the efficacy of the data derived from the current format of the census question on religion and identity in general. With growing demands for comprehensive planning of social service needs, the necessity for accurate data is more important than ever. Although much of this can be derived from the Census, there continues to be a key role for community-wide surveys. Copyright © 2005 John Wiley & Sons, Ltd.