Ostsiedlung (German pronunciation: [ˈɔstˌziːdlʊŋ], literally east settling), in English called the German eastward expansion, was the medieval eastward migration and settlement of German-speaking people from the Holy Roman Empire, especially its southern and western portions, into less-populated regions of eastern Central Europe and western Eastern Europe. The affected area roughly stretched from Slovenia in the south to Estonia in the north, and extended into Transylvania in the southeast. In part, Ostsiedlung followed the territorial expansion of the Empire and the Teutonic Order.
According to Jedlicki (1950), in many cases the term "German colonization" does not refer to an actual migration of Germans, but rather to the internal migration of native populations (Poles, Hungarians, etc.) from the countryside to the cities, which then adopted laws modeled on those of the German towns of Magdeburg and Lübeck. German historians of the nineteenth and twentieth centuries often exaggerated the importance of the adoption of Salic law and settlement in Central and Eastern Europe for political reasons. While the phenomenon did increase the economic wellbeing of destination countries, at least some of them, like medieval Poland, were already quite developed economically and politically in their own right,[under ] and the local Slavic population was already far more strongly established in its towns than previously believed: the whole process took part in territories where stable Slavic organisational structures already existed.
Before and during the time of German settlement, late medieval Central and Eastern European societies underwent deep cultural changes in demography, religion, law and administration, agriculture, settlement numbers and structures. Thus Ostsiedlung is part of a process termed Ostkolonisation ("east colonization") or Hochmittelalterlicher Landesausbau ("high medieval land consolidation"), although these terms are sometimes used synonymously.