Do you have ancestors from Schleswig and Holstein? You probably are aware that most parts of today’s Schleswig-Holstein were under Danish rule until becoming Prussian after the Second War of Schleswig in 1864/1867. Therefore, the inhabitants were listed in the Danish Censuses which are available online at the website of the Dansk Demografisk Database of the Danish Rigsarkivet.
You need to search the counties of Sydslesvig and Holsten and Lauenburg.
Note that some towns that today are German are listed in counties that today belong to Denmark, so do be sure to check the counties of Tønder (Tondern) (where you will find the North Frisian Islands), Haderslev (Hadersleben), Aabenraa (Apenrade), Ribe (Ripen) and Sønderborg (Sonderburg) as well if you can’t find them under Sydslesvig!
The northern part of Schleswig (Nordslesvig) had also become Prussian but returned to Denmark after a plebiscite in 1920. Today, this is the Danish County of Southern Jutland (Sønderjyllands Amt), in German called Nordschleswig.
If you have difficulties finding the Danish and Frisian name for a German town, this link might be helpful.
Just in case you’re a newbie to Danish research Rigsarkivet’s website holds a lot of interesting information on how to start if you click on this link!