A method of preventing instability in a vehicle-trailer combination (10) is provided. The method comprises monitoring relative movement between the vehicle (20) and the trailer (30) using optical sensing means; determining a trailer yaw condition in dependence on a signal output from the optical sensing means; comparing the determined trailer yaw condition to a predetermined threshold yaw condition; and when the determined trailer yaw condition is equal to or greater than the predetermined threshold yaw condition, outputting an alert signal indicative of the onset of instability in the vehicle-trailer combination.