Historical research on a site involves evaluating old aerial photographs, maps and archive material from former business occupants and government agencies. This usually enables the history of a site's usage to be very thoroughly reconstructed. The research reveals what production and other processes were carried out on the site in the past, as well as where – and to what extent – specific environmentally hazardous materials were employed. This provides us with indications of potential contamination points. The result of the historical research (according to DIN ISO 10381-5) is a contamination hypothesis, which then enables targeted, effective and cost-optimised technical investigation.