Over fifty years of aerial photography have added an enormous amount of information about archaeological sites and current reconnaissance continues to provide more. So that the maximum amount of information from aerial photographs can be obtained a National Mapping Programme (NMP) for England has been developed by the Royal Commission on the Historical Monuments of England (RCHME). This programme has developed from projects which mapped the cropmarks of the Yorkshire Wolds and four pilot project areas, jointly funded with English Heritage in the 1980s. These projects mapped the counties of Kent and Hertfordshire and two archaeologically important areas, the Thames Valley and the Yorkshire Dales. This paper provides a background to the National Mapping Programme and presents the results of two of the pilot projects, Hertfordshire and the Yorkshire Dales.