A coupling device including first and second rotatable bodies (1, 3), a plurality of engagement members (27,28) for selectively coupling the first and second rotatable bodies together to transfer drive between the rotatable bodies, and a guard device (2) for preventing the engagement members from coupling the rotatable bodies in certain predetermined operational conditions that include certain relative rotational positions of the rotatable bodies. The coupling device can be used in transmission systems. For example, such a transmission system can include first and second drive shafts (1, 7), first and second gear sets (15, 17) mounted on the shafts for transferring drive between the shafts, each gear set including a first gear wheel (3, 5) mounted o n the first shaft for rotation relative to the first shaft said first gear wheel having a plurality of drive forinations (19, 21), and a second gear (9, 11) mounted on the second shaft for rotation with the second shaft, selector means (13) for selectively transferring drive between the first shaft and either the first or second gear set including a plurality of engagenient members (28, 30) for engaging the drive formations, and a guard device (2, 102, 202, 302) for preventing the engagement members from engaging the drive formations in certain predetermined operational conditions that include certain relative rotational positions of the drive formations and the engagement members.