The opportunities opened by the use of cryoresistive and superconducting materials in underground transmission systems have led C.G.E. and L'Air Liquide to undertake, in close cooperation, a cryocable program which started in 1966. A first set of problems associated with the development of cryogenic cables deals with the cable system: design, safety, terminal equipment including leads, cryogenic equipment, refrigerators, and problems related to overload capability and reliability. A second set concerns the cable itself, i.e., scientific and technological problems associated with the conductor, the electrical insulation, and the thermal exchange between conductor and helium. We gained useful experience on the design problems and on the technological problems involved in the construction of a cryoconducting cable. A 20‐m aluminum cable cooled down to 25°K with pressurized helium flow was built and tested with 3500‐A dc under 20 kV; results are presented. On this model we solved the following types of problems. First, mechanical problems concerning cooling down of the cable, thermal contraction of the pipes, electrical insulation and conductors, construction of an invariable cable constituted by elementary helicaly wound conductors were solved. Second, thermal problems of reduction of heat leaks, conception of thermal insulation, and segmentation of vacuum jackets were solved. Third, electrical problems of design of 300°–‐25°K leads were solved; this problem of losses at both ends is, in proportion, more important for short model than for long cable. Finally, refrigeration problems of helium and nitrogen flows, thermal shields and design of refrigerators (optimal capacity and spacing) were solved. In order to solve problems concerning the cable itself, research has been done on superconducting materials, electrical insulation and heat exchange. Surface losses and critical currents of superconductors were compared. The losses mainly depend on the environment of conductors (electrical and thermal interface impedance between superconductor and stabilizing material, thermal impedance of the conductor versus helium, geometric irregularity of layers leading to transverse magnetic fields···). Results were presented on electrical insulation. A satisfactory dielectric is a key factor in a high‐capacity cable system. Measurements of the dielectric strengths of plastic tapes, immersed in liquid and hypercritical helium, have been made; the values obtained for 100‐&mgr; tapes are about 100 kV/mm and are similar with ambient temperature values. Breakdown strength has been measured for different electrode spacings and at various pressures. Dielectric losses are responsible for important thermal losses in cables; loss tangents of several materials have been measured at various temperatures and frequencies; the values decrease down to 5×10−6. Vacuum is not only a thermal insulation medium but also an electrical insulation environment. Dielectric strength and field emission currents have been measured at cryogenic temperatures for vacuum spacings and solid insulation in vacuum. The laws of thermal transfer between conductor and helium were studied for conventional configurations. Exchange coefficients were calculated and correlations between these coefficients and pressure drop were established. This experience is now used for the design of a superconducting cable. Several dc and ac cable configurations are under study. One of these systems consists of coaxial cables, in a pressurized helium pipe, with multilayer impregnated helium insulation. Another consists of pipes and multilayer insulation under vacuum. Preliminary calculations were made in order to optimize geometrical and electrical features of a superconducting ac cable. The values of electric field, current density, voltage, and ratio between coaxial cable diameters can be chosen to obtain a minimum value of the sum of costs relative to dielectric losses, reactive power compensation, and cable cost. Design studies of a superconducting high‐power cable are now going on; research and tests on superconducting materials, insulating materials, heat exchange, refrigeration and safety will be continued. Our aim is to design a model a few tens of meters long which will be built in 1972 to show that many of the problems involved in the construction of superconducting cables in the range of several thousand MVA have been solved.