ON APRIL 1st, 1969, the world's first jet V/S.T.O.L. weapons system, the Hawker Siddeley Harrier G.R. Mk. 1, entered military service with the Royal Air Force. This was the culmination of eight years of flight testing during which the Harrier evolved from the original Hawker P.1127 project of the late fifties. In this period an unparallelled record of jet V/S.T.O.L. flying has been built up, the Harriers alone flying some 1,700 hours of test flights with over 4,500 transitions, and their predecessors, the P.1127s and Kestrels, over 1,200 test and evaluation flying hours with 2,600 transitions. (Fig. 1). They have taken off and landed in small forest clearings by day and night, in central London and downtown New York, on ships decks ranging from U.S.S. Independence's 1,050 ft. × 250 ft. (310 m. ×75m.) down to H.M.S. Blake's 120ft. × 50ft. (36m. × 15m.), and from concrete, from grass and from aluminium pads. The comprehensive development programme has confirmed the engineering success of the design and proved that the Harrier jet V/S.T.O.L. weapons system is competitive and cost‐effective.