I love documents, so having a whole new set of documents to explore is exciting. What I am referring to is the fact that the 1939 Register of England & Wales is available online, and has been since February 2016. So, what makes it so exciting?
Well, for one thing, nobody was expecting it to be available until 2040, 100 years after it was compiled, so it’s 25 years earlier than expected. For another, it was used for various purposes. Initially, it was used as the basis for issuing national identity cards for the 
duration of the war (although they were not phased out until 1952 – my two older sisters had them, but not me). As such, the Register includes dates of birth which are often difficult to find without forking out a bundle for an official birth certificate.
Unlike a census, it was also a working document, and changes were made to it over time. For example, you will often see entries for women who married or divorced and changed their names as a result, so you may get hints as to the spouse’s last name 
written right into the Register. See, for example, this page. The information written in red and green makes it so much easier to find Norah’s marriage to William E McTrusty, and Phyllis’s marriage to James H Smith in the marriage index than looking through a bunch of entries and wondering who the spouse might be.
It was also used as the basis of the National Health Service Central Register long after the 1951 census came along. Remember, censuses were not working documents, which underlines the fact this was not a census. For some interesting information about the 1939 Register and its various uses and idiosyncracies click see this article from the Lost Cousins website.
At first, the Register was only available through Find My Past, but in February 2018, My Heritage allowed searches (but no images), while Ancestry made both available a few months later. Click one of the above links to go straight to the search page for the 1939 Register for each site.
Family History research in England 
Thanks so much for this post – I didn’t know of the register’s existence until I read it about it here! I’ve been having fun finding my relatives, but I have a question for you.. Where a record has been ‘closed’ does that mean the person died during the war? I was unable to locate my uncle and noted that there was a closed record at my Grandmother’s address, which I suspect might be him.
The closed message means that the stated date of birth is less than 100 years ago as of 1 January of the current year and no proof has been submitted for a death. I won’t be able to see my father’s entry until the start of 2021 because he was born in 1920. Because he died outside the UK I would have to provide a death certificate together with an affidavit in support of my request to open his record.
Ah, in that case there are some errors, as I have found a record for a relative who is still alive..
Thank you for the feedback. As you say, there are some errors. It would be nice to think that there are none, but I certainly hope that there are not too many of the type you describe.