Southern Schleswig (German: Südschleswig or Landesteil Schleswig, Danish: Sydslesvig) denotes the southern half of the former Duchy of Schleswig  on the Jutland Peninsula. The geographical area today covers the large area between the Eider river in the south and the Flensburg Fjord in the north, where it borders Denmark. Northern Schleswig, congruent with the former South Jutland County.
The Schleswig lands north of the Eider river and the Bay of Kiel had been a fief of the Danish Crown since the Early Middle Ages. The southern Holstein region belonged to Francia and later to the Holy Roman Empire, it was however held as an Imperial fief by the Danish kings since the 1460 Treaty of Ribe.
The Schleswig-Holstein Question at first culminated in the course of the Revolutions of 1848, when from 1848 to 1851 revolting German-speaking National liberals backed by Prussia fought for the separation of Schleswig and Holstein from Denmark in the First Schleswig War. Though the status quo was restored, the conflict lingered on and on 1 February 1864 the Prussian and Austrian troops crossed the Eider sparking off the Second Schleswig War, after which Denmark had to cede Schleswig and Holstein according to the Treaty of Vienna. After the Austro-Prussian War of 1866, victorious Prussia took control over all Schleswig and Holstein but was obliged by the Peace of Prague to hold a referendum in predominantly Danish-speaking Northern Schleswig, which it never did.
After the German defeat in World War I the Schleswig Plebiscites were decreed by the Treaty of Versailles, in which the present-day German-Danish border was drawn taking effect on 15 June 1920, dividing Schleswig in a southern and northern part and leaving a considerable Danish and German minority on both sides 
Southern Schleswig is part of the German state (Bundesland) of Schleswig-Holstein, therefore its denotation as Landesteil Schleswig. It does not however form an administrative entity, but consists of the districts (Landkreise) of Schleswig-Flensburg, Nordfriesland, the urban district (Kreisfreie Stadt) of Flensburg and the northern part of Rendsburg-Eckernförde.
Beside Standard German, Low Saxon dialects (Schleswigsch) are spoken, as well as Danish (Standard Danish or South Schleswig Danish) and its South Jutlandic variant, furthermore North Frisian in the west. Danish and North Frisian are official minority languages. Many of the inhabitants who only speak German and not Danish do not consider the region any different from the rest of Schleswig-Holstein. This notion is disputed by those defining themselves as Danes, South Schleswigans or Schleswigans, particularly historians and people organised in the institutions of the Danish minority of Southern Schleswig, such as the South Schleswig Voter Federation. Many of the Last names found in the region are very often of Scandinavian or Danish form, with the -sen endings like Petersen.
- Lars Henningsen: Sydslesvigs danske historie, Flensborg 2013.
- Lars Henningsen: Zwischen Grenzkonflikt und Grenzfrieden, Flensburg 2011 (as pdf document)
- Karen Margrethe Pedersen: Dansk sprog i Sydslesvig: det danske sprogs status inden for det danske mindretal i Sydslesvig, Institut for grænseregionsforskning Aabenraa 2000
- Kathrin Sinner: Schleswig-Holstein - das nördliche Bundesland: Räumliche Verortung als kulturelles Identitäskonstruk, page 86
- Sønderjylland A-Å, Aabenraa 2011, page 364
- Region Sønderjylland-Schleswig: Politische Entwicklungen im Mittelalter
- Region Sønderjylland-Schleswig: Nationale Entwicklung im 19. Jahrhundert (German)
- vimu - Det Virtuelle Museum: Genforening (Danish)
- vimu - Das Virtuelle Museum: Volksabstimmung (German)
- Region Sønderjylland-Schleswig: Sprachen und Dialekte südlich der Grenze(German)