ABSTRACT OF THE DISCLOSURE A two-speed transmission unit which couples a rotatable input shaft to a rotatable output shaft. A gear unit includes-a sun gear fixed to the input shaft and rotatable therewith. One or more planet gears are disposed for orbital motion around the sun gear, and the planet gears are carried by a planet carrier which orbits with the planet gears about the sun gear. First clutch means is provided for coupling the sun gear to the output shaft for rotating the output shaft at a first angular velocity equal to the angular velocity of the sun gear, and second clutch means is provided for coupling the planet carrier to the output shaft for rotating the output shaft at a second angular velocity equal to the angular velocity of the planet carrier about the sun gear. Both the first and second clutch means are continually biased into engaged condition, and means are provided for (1) selectively placing one clutch means in a disengaged condition and allowing the biasing means to place the other in engaged condition, or (2) placing both clutch means in a disengaged condition, thereby allowing "free wheeling" of the input shaft. There is also provided a fail-safe brake to lock the input and output shafts against rotation when neither clutch means is placed in its disengaged condition.