- Codes and Ciphers in the Second World War: http://www.codesandciphers.org.uk/. Scroll down past the ads to get to the substance.
- Decoding Nazi Secrets: http://www.pbs.org/wgbh/nova/decoding/. This is the companion site to the two-hour NOVA program on how the Allies cracked the Enigma.
- Englandspiel by Adri Wijnen: http://www.englandspiel.nl/index.php?taal=en
- Military Intelligence Museum, Shefford, U.K.: http://www.armymuseums.org.uk/museums/0000000048-Military-Intelligence-Museum.htm
- Netherlands Intelligence Studies Association: http://www.nisa-intelligence.nl/
- Plan Sussex: http://www.plan-sussex-1944.net/. English language: http://www.plan-sussex-1944.net/anglais/macro.htm
