Abstract: The invention relates to an anti-jackknife apparatus for a tractor-trailer having a fifth wheel on the tractor unit to which the trailer is coupled. The fifth wheel is adapted to permit the trailer to be coupled thereto so that the trailer can turn with respect to the tractor unit when the tractor-trailer is in normal use. Braking means are provided in association with the fifth wheel and are adapted, when operated, to restrict turning of the trailer with respect to the tractor unit in use. Control means for the braking means are provided and are adapted to be operated from the driving cab of the tractor unit whereby, in use, the braking means can be used to arrest or prevent uncontrolled swinging of the trailer with respect to the tractor unit.