While still a hereditary prince of Hesse-Kassel, Wilhelm I (1743-1821) was an enthusiastic collector of coins and medals. He did not give up this passion as a reigning Elector. His collection was built up in Hanau. Taking office, Wilhelm I took the coins and medals with him to Kassel. When the Electorate of Hesse-Kassel was annexed by Prussia in 1866 the collection became public museum property but was sold in the 1920s. Thus, it is not preserved in its entirety, but the Kestner Museum in Hanover acquired a very large part of the Hanau medals at the 1927 auctions, so that the collection can be partially reconstructed. A reconstructed compilation of its holdings offers insight into the history of the coin trade because Wilhelm I built up his collection with the help of the banker Mayer Amschel Rothschild, who is considered the first coin dealer.