Nation-wide identification systems, such as the National Registration implemented in the UK in 1939 at the start of World War II. Use this tag for questions about the 1939 National Register and identity cards, or any similar ID that was enacted at a national level.

On passage of the National Registration Act 1939 at the start of World War II, a national register was made and identity cards issued to UK citizens. During the war, everybody had to carry their identity card. National Registration was maintained after the war ended, until the Act was repealed in 1952. The National Register was the basis for the identification system used by the National Health Service (NHS) when it was formed in 1948.

The 1939 Register is seen as a "census substitute", although it was not a census, as it is a comprehensive record of the civilian population in 1939. It is particularly important because the 1931 census was completely destroyed by a fire, and there was no 1941 census due to the war.