The specification discloses a modular system for the formation of a prosthesis for the replacement of a long bone in a human or animal body. The system provides a prosthesis comprising a proximal or distal end component such as a trochanter (1) or condylar head, forming one component of a prosthetic joint, a shaft (5) for assembly with the distal or proximal end component and a stem (8) for engagement in a resected bone. The system allows the assembly of a custom fitted prosthesis from a limited range of components.