A guided underwater object (GUO) (100) comprises steering means, an on-board guidance system (200) for controlling the steering means, and a hydrophone unit or units (104A, 104B, 104C, 104D, 104E, 104F, 104G, 104H) arranged to pass signals induced therein by an acoustic guiding beam to the on-board guidance system to effect directional control of the self-propelled underwater object. The GUO is guided by a remote guidance system characterized by a plurality of transmitter arrays (402A, 402B, 402C, 402D; 412A, 412B, 412C, 412D; 422P, 422Q, 422R) arranged to transmit a plurality of contiguous acoustic beams (302A, 302B, 302C, 302D) each having a different frequency or frequency chirp (or other acoustic signal coding) and enclosing a guidance corridor of lower sound level (`null`) (304) extending approximately in the direction of propagation of the acoustic beams and defining a flight path of the object. The GUO requires no on-board search sensors, no communications link with the guidance system and is of low cost.