Abstract: A joint prosthesis for a human or animal body, which comprises first (20) and second (40) components each made of bio-compatible material and adapted for secural to respective bones (13,14) of the body in the vicinity of a natural joint (16) after surgical excision of the natural joint (16). The components (20,40) of the prothesis (12) are unattached to each other and, in use, are held together to form the joint by the natural ligaments, tendons and muscles of the body in the vicinity of the natural joint (16). The first component (20) comprises a stem (21) for reception by a bone cavity and a bulbous head (22) presenting a three-dimensionally curved convex articulation surface (23) which in the sagittal plane has a radius of curvature which changes within the arc length of the surface in that plane and which has a length of at least 90.degree..