A method of controlling the braking of a coupled tractor-trailer combination in which the tractor includes a drive train having an engine and at least one ground-engaging member driveably connected thereto via an adjustable-ratio transmission; at least one ground-engaging member of the tractor includes at least one tractor brake; and at least one ground-engaging member of the trailer includes at least one further trailer brake, the method comprising the steps of determining a reference deceleration value AX_REF derived from (i) the deceleration of the tractor, when considered alone, resulting from braking effort applied via the tractor brake and/or (ii) the deceleration of the tractor, when considered alone, resulting from engine braking; wherein said reference deceleration value is calculated in accordance with the expression A X _ REF = F TRACTOR _ BRAKES M TRACTOR and wherein AX_REF is the reference deceleration value; F TRACTOR_BRAKES is the brake force generated by the tractor from the braking effort applied via the at least one tractor brake and/or by engine braking; and M TRACTOR is the mass of the tractor omitting the mass of the trailer; the method including using the reference value as a control input in an open-loop control to control the braking effort of the trailer brake so that the braking of the tractor-trailer combination approximates to the braking of the tractor when considered alone; said method being characterized by including the steps of determining from the deceleration value, in accordance with a predetermined relation, a target value of the braking effort of the trailer brake, wherein the target value of the braking effect of the trailer brake is F TRAILER BRAKES OPEN LOOP and is related to the reference deceleration value A X_REF by the expression: A X _ EB _ OPEN _ LOOP = F ENGINE _ BRAKES + F TRAILER _ BRAKES _ OPEN _ LOOP M TRACTOR + M TRAILER ≅ A X _ REF in which additionally: A X EB OPEN_LOOP is the deceleration of the tractor-trailer combination resulting from activation of at least one said trailer brake in dependence on the said target value; F ENGINE_BRAKES is the braking effort resulting from engine braking; and M TRAILER is the mass of the trailer forming part of the combination. and wherein AX_REF is the reference deceleration value; F TRACTOR_BRAKES is the brake force generated by the tractor from the braking effort applied via the at least one tractor brake and/or by engine braking; and M TRACTOR is the mass of the tractor omitting the mass of the trailer; the method including using the reference value as a control input in an open-loop control to control the braking effort of the trailer brake so that the braking of the tractor-trailer combination approximates to the braking of the tractor when considered alone; said method being characterized by including the steps of determining from the deceleration value, in accordance with a predetermined relation, a target value of the braking effort of the trailer brake, wherein the target value of the braking effect of the trailer brake is F TRAILER BRAKES OPEN LOOP and is related to the reference deceleration value A X_REF by the expression: A X _ EB _ OPEN _ LOOP = F ENGINE _ BRAKES + F TRAILER _ BRAKES _ OPEN _ LOOP M TRACTOR + M TRAILER ≅ A X _ REF A X EB OPEN_LOOP is the deceleration of the tractor-trailer combination resulting from activation of at least one said trailer brake in dependence on the said target value; F ENGINE_BRAKES is the braking effort resulting from engine braking; and M TRAILER is the mass of the trailer forming part of the combination.