The Orthodox Defence to the Queen's Gambit is an extremely resilient system for Black, and a solid favorite of many world champions. As Black's main ideas are easily grasped, it is also a very popular option at club level. In this, the first major work on the subject, the flexible and powerful move 5 Bf4! is explained. While far less hackneyed than the standard systems in which the bishop goes to g5, the move denies Black the standard simplifying manoeuvres and sets an entirely new set of problems.