An instrument drive unit for use in a robotic surgical system, the instrument drive unit comprising: a carriage configured to be coupled to a robotic arm; a plurality of drive shafts rotationally supported in the carriage; a plurality of electric motors disposed about the plurality of drive shafts, each electric motor of the plurality of electric motors including a stator and a rotor disposed within the stator; and a plurality of drive gears, each drive gear of the plurality of drive gears fixed to a corresponding drive shaft of the plurality of drive shafts and configured for interfacing with a corresponding driven member of the electromechanical surgical instrument, wherein each rotor is configured to rotate a corresponding drive gear of the plurality of drive gears in response to an activation of a respective electric motor of the plurality of electric motors to actuate a function of the electromechanical surgical instrument wherein each stator is fixed relative to the carriage, and each rotor is rotatable relative to and within a corresponding stator characterized in that the plurality of electrical motors are vertically stacked within the carriage, and further comprising a plurality of ring gears, each ring gear of the plurality of ring gears fixed to a corresponding rotor and operably coupled to a corresponding drive gear of the plurality of drive gears wherein each ring gear of the plurality of ring gears is concentrically disposed within a corresponding rotor, such that rotation of the rotor results in a rotation of the corresponding ring gear of the plurality of ring gears wherein the plurality of ring gears are vertically stacked within the plurality of electric motors. a carriage configured to be coupled to a robotic arm; a plurality of drive shafts rotationally supported in the carriage; a plurality of electric motors disposed about the plurality of drive shafts, each electric motor of the plurality of electric motors including a stator and a rotor disposed within the stator; and a plurality of drive gears, each drive gear of the plurality of drive gears fixed to a corresponding drive shaft of the plurality of drive shafts and configured for interfacing with a corresponding driven member of the electromechanical surgical instrument, wherein each rotor is configured to rotate a corresponding drive gear of the plurality of drive gears in response to an activation of a respective electric motor of the plurality of electric motors to actuate a function of the electromechanical surgical instrument wherein each stator is fixed relative to the carriage, and each rotor is rotatable relative to and within a corresponding stator characterized in that the plurality of electrical motors are vertically stacked within the carriage, and further comprising a plurality of ring gears, each ring gear of the plurality of ring gears fixed to a corresponding rotor and operably coupled to a corresponding drive gear of the plurality of drive gears wherein each ring gear of the plurality of ring gears is concentrically disposed within a corresponding rotor, such that rotation of the rotor results in a rotation of the corresponding ring gear of the plurality of ring gears wherein the plurality of ring gears are vertically stacked within the plurality of electric motors.