The Nimzo-Indian is an extremely popular and reliable defence, upon which most world champions have relied at some point in their careers. Black controls the vital e4-square with his pieces, and retains total flexibility with his central pawns. By playing 4 f3, White challenges Black's idea head-on. The result is sharp, forcing play of a type that is rare in the early stages of most Nimzo-Indians. The 4 f3 Nimzo enjoyed a period of great popularity in the early 1990s, following a number of spectacular victories by the teenage Alexei Shirov. However, good responses were found and the line's popularity declined. A number of players quietly worked away at revitalizing 4 f3, and it is now once again a potent weapon. Following its period out of the limelight, it has now regained its surprise value, which will be greatly valued by club players wishing to spring 4 f3 on their opponents.