From the 3rd September 1939 to 31st May 1943 the Allies lost 11576000 tonnes of shipping. Although the great destructive power of U-boats had been demonstrated during the First World War the essential lesson to be learnt (the need for huge air and sea escort forces) was incapable of being implemented at the outbreak of the Second World War. The expectation that ASDIC (a submarine location system developed by the Admiralty during the interwar years) would prove immediately effective and prevent a repetition of the (1915-1918) U-boat offensive was not realised. There was an urgent need for technology to aid the air and sea forces in their searches for submarines. Notwithstanding the development of airborne and shipborne radars, the breaking of the German naval enigma cyphers, the utilisation of land-based and ship-based HF direction finders, HF radio telephony, Leigh lights, improved ASDICs, operational research and many other innovations the threat to the security of the United Kingdom in March 1943 was, according to the Admiralty, greater than at any other rime of the war. But in May 1943 the U-boats were withdrawn temporarily from the North Atlantic following the introduction of a new radar, ASV Mark III. The paper is based on material held in the Public Record Office and highlights the application of technology in the Battle of the Atlantic.