A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising: an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis； a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier； a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs； a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear； a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon； and an intermediate gear train borne by the carrier and coupling the second shaft gear to the second drive gear, the intermediate gear train comprising an intermediate shaft arranged to rotate about an axis parallel with the first rotation axis, the intermediate shaft having a third shaft gear thereon, the third shaft gear being arranged to engage the second drive gear.
