A wheeled vehicle (2) incorporating first and second wheel assemblies (7a,7b) which are provided respectively at laterally spaced apart positions on the vehicle (2). Each wheel assembly (7a,7b) is rotatably mounted to the vehicle for rotation about a respective orientation axis (X 1 ,X 2 ). Each wheel assembly has at least one wheel (13a,13b,14a,14b) which is attached rotatably to a mounting device (9a,9b) for rotation about a roll axis (A 1 ,A 2 ). The or each wheel having a radius (R), each wheel assembly having a track width (W) which is substantially equal to or greater than the radius (R). The roll axes (A 1 ,A 2 ) of the first and second wheel assemblies (7a,7b) are substantially parallel and co-axially aligned with one another in a normal orientation when the vehicle (2) travels over a roll plane (P) in a stable condition. If the vehicle (2) becomes unstable when in use such that one of the wheel assemblies (7a,7b) depart from the roll plane (P), a rotational torque is generated across the track width (W) of the other wheel assembly (7a,7b) in contact with the roll plane (P). The torque causes the said other wheel assembly (7a,7b) to rotate in a predefined arc (M2) about its orientation axis (X 1 ,X 2 ) in a direction away from its normal orientation to stabilise the vehicle (2).