A hybrid hydraulic drive system for a vehicle comprises a prime mover, a transmission connecting the prime mover to drive wheels, a fluid energy storage accumulator, and a reversible hydraulic machine in fluid communication with the energy storage accumulator and drivingly coupled to the prime mover upstream of the transmission. The hybrid hydraulic drive system is arranged such that in a retarding mode the reversible hydraulic machine retards the drive wheels of the vehicle by pumping fluid into the accumulator. In a driving mode the reversible hydraulic machine supplies a supplementary power to the drive wheels of the vehicle using the pressurized fluid from the accumulator to assist propulsion of the vehicle. In a neutral mode the reversible hydraulic machine is disconnected from the prime mover to render the reversible hydraulic machine substantially inoperative to exert any significant driving or retarding influence on the drive wheels.