The axes of rotation of the thumb interphalangeal and metacarpophalangeal joints were located using a mechanical method. The interphalangeal joint axis is parallel to the flexion crease of the joint and is not perpendicular to the phalanx. This offset of the axis with respect to the phalanx explains the ulnar deviation and pronation that occurs with flexion of the interphalangeal joint. The metacarpophalangeal joint has 2 fixed axes: a fixed flexion-extension axis just distal and volar to the epicondyles, and an abduction-adduction axis related to the proximal phalanx passing between the sesamoids. Neither axis is perpendicular to the phalanges. All physiologic motion for these joints occurs about the axes. These are the mechanical axes of the joints through which the muscles and external forces act. Knowledge of their location should help in constructing prosthetic joints and in planning reconstructive surgery such as tendon transfers.