Lebensraum (help·info) (German for "habitat" or literally "living space") was one of the major genocidal political ideas of Adolf Hitler, and an important component of Nazi ideology. It served as the motivation for the expansionist policies of Nazi Germany, aiming to provide extra space for the growth of the German population, for a Greater Germany. In Hitler's book Mein Kampf, he detailed his belief that the German people needed Lebensraum ("living space", i.e. land and raw materials), and that it should be found in the East. It was the stated policy of the Nazis to kill, deport, or enslave the Russian, Ukrainian, Polish and other Slavic populations, whom they considered inferior, and to repopulate the land with Germanic people. The entire urban population was to be exterminated by starvation, thus creating an agricultural surplus to feed Germany and allowing their replacement by a German upper class.

The idea of a Germanic people without sufficient space dates back to long before Adolf Hitler brought it to prominence. Through the Middle Ages, German population pressures led to settlement in Eastern Europe, a practice termed Ostsiedlung. The term Lebensraum in this sense was coined by Friedrich Ratzel in 1901, and was used as a slogan in Germany referring to the unification of the country and the acquisition of colonies, based on the English and French models, and the westward expansion of the United States. Ratzel believed that the development of a people was primarily influenced by their geographical situation and that a people that successfully adapted to one location would proceed naturally to another. These thoughts can be seen in his studies of zoology and the study of adaptation. This expansion to fill available space, he claimed, was a natural and necessary feature of any healthy species.