The OSCAR system for estimating audiences of individual poster sites was developed in the UK for the outdoor advertising contractors from 1981 and launched in October 1985. From a census of all poster sites audiences were modelled in the form of gross vehicular and pedestrian passages per week. By taking account of the visibility of each site or panel, gross audiences can be netted down to gross Opportunities to See (OTS). OSCAR therefore provides comprehensive information on the size of the audience for individual posters, but does not give information on audience structure or composition. The National Poster Survey undertaken in October 1987 complements OSCAR data by providing information on all poster passages by a national sample of adults with respect to a national sample of poster panels. It generates estimates of the structure of the national poster audience in terms of frequency and coverage, and of its composition in terms of demographics. In conjunction with OSCAR it provides the means of modelling the audience characteristics of campaigns based on specified sites. This paper describes how the survey was developed and undertaken, the results obtained and how they will be used in practice to enhance the value of the OSCAR data in the planning of poster campaigns.