A hydraulic energy storage system (comprising a hydraulic pump/motor, a high pressure hydraulic accumulator, a low pressure hydraulic accumulator/reservoir, and interconnecting hydraulic lines) is incorporated into a EV, HEV, or PHEV to provide hydraulic regenerative braking and propulsive assistance for the vehicle. Implementation of the low cost and long-lasting hydraulic energy storage system in the vehicle, together with the electric energy storage system (comprising a motor/generator and battery pack) of the vehicle, allows significantly reduced demands and higher operating efficiencies for the battery pack, thereby facilitating a more cost-effective, efficient and/or durable overall energy storage system for the vehicle.