A hub comprises a cylindrical, hollow hub member (2), which is mounted to rotate about its axis and in whose interior is a transmission system having an input (10), which is mounted to rotate about the axis, and an output connected to rotate with the hub member. The transmission system comprises first and second epicyclic gearsets (14, 16). The first gearset (14) comprises a first sun gear (18), which is mounted to rotate about the axis and is in mesh with a plurality of first planet gears (20) mounted to rotate about respective planet shafts (22) carried by a first common carrier (24), which is mounted to rotate about the first axis. The second gearset (16) comprises a second sun gear (28), which is mounted to rotate about the axis and is in mesh with a plurality of second planet gears (30) mounted to rotate about respective planet shafts (32) carried by a second common carrier (34). The first and second sun gears (18, 28) are connected to the rotors (38, 40) of first and second electric motor/generators, respectively. The power connections of the two stators (42, 46) are connected via a controller (50) arranged to control the transmission of power from one motor/generator to the other.