A robot arm 1 comprises a plurality of limbs 3 articulated relative to each other, the arm 1 extending from a base 2 to a distal limb 3c, 403 carrying a tool 9 or an attachment point 8 for a tool 9, the distal limb 3c, 403 being attached by a revolute joint to a second limb 3b, 440 and the arm 1 comprising a motor 406 having a body and a drive shaft 414 arranged for driving rotation of the distal limb 3c, 403 relative to the second limb 3b, 440 about the revolute joint, the body of the motor 406 being fast with the distal limb 3c, 403. The motor 406 may drive a pinion gear 415 engaged with an internally toothed gear 420 for driving rotation of the distal limb 3c, 403 relative to the second limb 3b, 440. A tool motor 407 may drive a driveshaft 440 for a tool or instrument.
