Eric Voegelin is famous as a philosopher of history and a as one of the most eminent political scientists of the 20th century. His most fundamental work on political theory, the "New Science of Politics, is nowadays considered a classic in ist field. While the "New Science" has always been a very controversial book, ist critics have hardly ever taken the pain to pinpoint the weaknesses they condemmed Voegelins book for. There is, however, one exception: Only shortly after the appearance of Voegelins "New Science" in 1954, Hans Kelsen has written a most detailed reply to this book of his former student. This reply, which was known to Voegelin and is mentioned in his autobiography, is now being published by the ontos verlag. Being a distinguished philosopher himself of an erudition and breadth of knowledge that matches that of Eric Voegelin, Hans Kelsen is able to support the critical stance he takes on Voegelin "New Science" by clear and well founded argument. This critical reply to Voegelins "New Science" is not only an important contribution to the dispute about the foundations of political order in modern society, but will also prove valuable to readers generally interested in Voegelins life and work.