Abstract: An arming unit is used on an aircraft to control the arming of a store (20) containing explosive on its release from the aircraft so that the store is not armed until after its release and after it has travelled a predetermined distance away from the aircraft. Such an arming unit comprises a pivoted hook (12) for engaging and retaining the ring or loop (26) on the end of an arming wire attached to an arming pin on the store (20), the hook being biased into closed position, an inter-connection (16) between the hook (12) and a spring loaded plunger (4) having an extending portion constituting a sensor for engaging the upper surface of a store (20) to be released, the inter-connection (16) serving to open the hook (12) as a result of downward movement of the plunger (4) on release of the store (20), and a lock (22, 46) for holding the hook (12) in its closed position and thus preventing operation of the inter-connection (16).