This chapter, dealing with the third phase in the reception of Klein’s “Erlangen Program,” can only touch on major currents connected with its place in the mathematics and physics of the early twentieth century. This necessitates taking a selective approach in order to gain a broad perspective on several large-scale shifts that took place over those years. For Felix Klein, the standard context in which he and other leading representatives of the German mathematical community plied their trade was the lecture hall and seminar.