The key measure of census quality is the level of response achieved. In recent censuses worldwide this level has been in the high nineties per cent. This was also true of censuses in Britain in 1991 (98 per cent). However, what was particularly noticeable about the census in Britain in 1991 was the differential response rate and the difficulty in effectively measuring this. The One Number Census programme was set up in the UK to research and develop a more effective methodology to measure and account for underenumeration in the 2001 Census. The key element in this process is the Census Coverage Survey – a significantly larger and redesigned post-enumeration survey. This article describes the planning and design of the survey with particular emphasis on the implementation of the proposed field methodology in practice. It also provides a highlevel overview of the success of the survey.