Ostsiedlung is the German word for the colonization of Europe beyond the boundaries of the already majority German areas. The Atlas of Medieval Europe published by Routledge has a section on it written by a gentleman associated with the University of St. Andrews in Scotland.
Between 1100 and 1350 eastern Europe was transformed by a wave of German immigration (the Ostsiedlung), which moved the eastern boundaries of the German-speaking world hundreds of miles beyond its former limit on the rivers Elbe and Saale. In some areas, such as Brandenburg, this new settlement came in the wake of conquest by German lords and knights, but in many other regions, such as Pomerania and Silesia, it was local Slav princes who encouraged German settlement. National antagonism was not important. The new settlers wanted land and the local rulers were happy to grant it and to profit, directly or indirectly, from the taxes, rents and tithes flowing from the new villages.
What I found intriguing was that, according to the author of this section, German settlement of Pomerania was not by conflict and conquest, but by invitation. The “local Slav princes . . . encouraged German settlement.” I have read other sources that have asserted this as well, although the specific details of how this played out is much more complicated and convoluted.
The frontier of settlement began to move in the first half of the twelfth century when immigration was actively promoted by such vigorous border lords as Adolf of Holstein, Henry the Lion, duke of Saxony, and Albert the Bear, margrave of Brandenburg. They advertised the attractions of the eastern frontier among the overcrowded inhabitants of western Germany and the Low Countries and soon streams of colonists were arriving in east Holstein, Schwerin, Ratzeburg and Brandenburg. The pace quickened in the thirteenth century as planned and large-scale development was undertaken in Pomerania and the Polish lands.
Where did my ancestors hail from before Pomerania? My uncle’s Y chromosome is grouped into what’s known as R1a, and likely R1a1a.
Rural settlement often involved the lay-out of entirely new villages, composed of standard, rectilinear farms (Hufen or mansi). The recruitment and organization of the colonists was the task of a planning entrepreneur (locator), who received land and privileges in the new settlement as his reward. Slav peasants were not usually dispossessed (though there are some instances of this), since in general there was plenty of land, especially for those willing to drain marshes or fell forests.
Rural settlement was complemented by new urban foundations. German burgesses formed the core of most of the new chartered towns founded in eastern Europe in these centuries. They brought their language, culture and law with them. Places as significant for German civilization as Lübeck, Berlin and Leipzig were twelfth- or thirteenth-century foundations in previously Slav landscapes.
German settlement was expansive.
German urban settlement spread far beyond the limits of German peasant settlement and up to the borders of Russia there were German burgesses, living according to German town law, in the midst of native rural populations.
In some regions German conquest and settlement coincided with conversion to Christianity. The Slavs who inhabited Mecklenburg and Brandenburg, for example, were pagan until the twelfth century. In most areas, however, Germans came to lands that were already Christian. But one German settlement was unique in being created and permanently maintained by holy war. This was the domain of the Teutonic Knights, Prussia and Livonia, where German crusaders brought forcible baptism to pagan Baltic peoples. By the fourteenth century, although the pagan Lithuanians were far from being defeated, a German population of landlords, churchmen, burgesses and (in Prussia) peasants had settled, from Danzig to the Gulf of Finland, under the rule of the crusading knights.
The end result of the Ostsiedlung was the Germanization of vast areas east of the Elbe and an increase in their economic productivity. Some of the political units created in the process, like Brandenburg and Prussia, were to have an important role in subsequent European history.
Recommendations for reading more about this subject include: The Making of Europe: Conquest, Colonization and Cultural Change, 950-1350 by Bartlett, The Northern Crusades: The Baltic and the Catholic Frontier, 1100-1525 by Christiansen, and Medieval Scandinavia by Sawyer and Sawyer.
ajh
AARON HILL’S NOTEBOOK 
I grieves me to read of your point of departure: “Between 1100 and 1350 eastern Europe was transformed by a wave of German immigration….” Yes, but why stop there and present the Germanic people as settlers, immigrants and non-natives?? Go back another 1000 years, and you find all the areas on the map labeled “Ostsiedlung” to be inhabited by the Germanic Goths, Ostrogoths, Wandalen and the like, with the Slavs nowhere to be found!
Germanics moved west continuously–Franks, Anglo-Saxons, etc. all the way to Scotland, Morocco, Sicily, Turkey etc, while vacating their homes in the East. In around 1000AD, they returned to regain their old homeland which by then was brimming with Slavs. Gradually they did until 1919–then it reversed once again.
So to be fair, push back the date of year 1 AD, and not 1100AD to get a more accurate and impartial picture.
By the way, before the Germanic folks settled the land, it was the Iranic people who lived from Manchuria to Brandenburg, from Prussia to Catalonia (Goth-Alania). They were the Alans, the Sarmatians, the Scythians (Ishkazai), the Saka, the Sorbs!