A boost system for providing boost pressure to an air intake manifold of an engine, the boost system comprising: a supercharger having rotors and a supercharger input shaft; an electric motor/generator (122, 422, 522, 622) including a drive shaft; a planetary gear set (126, 621) for transferring torque between the supercharger input shaft, the electric motor/generator and a crankshaft of the engine, the planetary gear set including a sun gear (156, 456, 556, 656), a ring gear (162, 462, 562, 662) and a carrier (158, 458, 558, 658) that carries planetary gears (160, 460, 560, 660), the carrier, the sun gear and the ring gear being configured to rotate about a central axis (165, 465, 565, 665) of the planetary gear set, and wherein the drive shaft of the electric motor/generator rotates about the central axis of the planetary gear set; characterized by: the system including a brake for braking rotation of the supercharger rotors, wherein: the electric motor/generator is axially in-line with the planetary gear set and the supercharger, and wherein the sun gear (156, 656) is coupled to the supercharger input shaft, wherein the carrier (158, 658) is coupled to a pulley (172, 672) that is coupled to the engine crankshaft, and wherein the electric motor (122, 622) is positioned along the central axis (165, 665) at a location between the sun gear (156, 656) and the pulley (172, 672), and wherein the carrier is coupled to the pulley by a torque transfer shaft aligned along the central axis of the planetary gear set, the drive shaft of the electric motor / generator being hollow and the torque transfer shaft passing through the drive shaft of the electric motor/generator, the torque transfer shaft and the drive shaft being rotatable relative to one another about the central axis of the planetary gear set, and the torque transfer shaft being adapted to rotate in unison with the carrier about the central axis of the planetary gear set. a supercharger having rotors and a supercharger input shaft; an electric motor/generator (122, 422, 522, 622) including a drive shaft; a planetary gear set (126, 621) for transferring torque between the supercharger input shaft, the electric motor/generator and a crankshaft of the engine, the planetary gear set including a sun gear (156, 456, 556, 656), a ring gear (162, 462, 562, 662) and a carrier (158, 458, 558, 658) that carries planetary gears (160, 460, 560, 660), the carrier, the sun gear and the ring gear being configured to rotate about a central axis (165, 465, 565, 665) of the planetary gear set, and wherein the drive shaft of the electric motor/generator rotates about the central axis of the planetary gear set; the system including a brake for braking rotation of the supercharger rotors, wherein: the electric motor/generator is axially in-line with the planetary gear set and the supercharger, and wherein the sun gear (156, 656) is coupled to the supercharger input shaft, wherein the carrier (158, 658) is coupled to a pulley (172, 672) that is coupled to the engine crankshaft, and wherein the electric motor (122, 622) is positioned along the central axis (165, 665) at a location between the sun gear (156, 656) and the pulley (172, 672), and wherein the carrier is coupled to the pulley by a torque transfer shaft aligned along the central axis of the planetary gear set, the drive shaft of the electric motor / generator being hollow and the torque transfer shaft passing through the drive shaft of the electric motor/generator, the torque transfer shaft and the drive shaft being rotatable relative to one another about the central axis of the planetary gear set, and the torque transfer shaft being adapted to rotate in unison with the carrier about the central axis of the planetary gear set.