A new technique has been developed for making superconducting weak links, making use of the fact that the superconducting transition temperature (Tc) of certain metals (e.g., W, Mo) can be greatly increased by ion implantation. By implanting controlled patterns of N+ and S+ ions into Mo films, structures have been produced in which two heavily doped Mo regions with high Tc are joined by a short (≲1 μm) lightly doped Mo region with lower Tc which acts as a weak link. These rugged and stable structures display the ac and dc Josephson effects. These techniques lend themselves to the production of superconducting integrated circuits containing weak links.