The purpose of this investigation was to define the action of each of the thenar muscles and to correlate their individual actions with the action of the group as a whole. The attachments of the thenar muscles to the extensor expansion has been emphasized because extension at the distal joint of the thumb is an important part of their action. The explanation given for the dynamic flexion deformity of the distal thumb joint, or Froment's sign, that appears with ulnar-nerve paralysis, is based upon the failure of the muscles innervated by the ulnar nerve to extend the distal phalanx of the thumb during pinch. The actions of the abductor brevis, flexor brevis, and opponens muscles in producing similar movements of the thumb, but to different degrees, are described in order to simplify the analysis of the disability that arises following division of the median nerve.