Large diameter (up to approx. 2 m) tritium compatible vacuum valves will be required for a broad range of applications around the NET/ITER torus. This paper focuses on the development steps and current design status of the valves located immediately upstream of the torus primary vacuum pumps. The number (24) and size (1500 mm nominal diameter) of these valves has been established in studies of the required particle exhaust rate from the torus and the conductance of the divertor duct and manifold system. The three principal functions are to isolate the torus during maintenance, to prevent back-streaming during regeneration of compound cryopumps, and to provide fast closure following accident or upset conditions. The design input parameters are tabulated in the paper. Initial engineering studies indicate that a gate valve is the preferred configuration to achieve low conductance losses and a design compatible with the confined space available. In order to meet the specified internal leak tightness (10−4 Pa.m3. s−1) in the potentially dust-laden environment, an elastomeric sealing material is recommended. This will keep the sealing forces and therefore the overall weight and dimensions of the valve within acceptable limits. Because of the arduous operating environment (dust, tritium, neutron activation and high operating frequency) provision will be made for change-out of the valve seals as a routine maintenance activity. A valve design in which the bonnet, stem and valve disc (along with the elastomer seal rings) can be removed from the valve body by a remotely operated manipulator and transferred to a centralized hot cell location for refurbishment has therefore been specified. Development of the valve includes both engineering studies and laboratory test work, and these are described in the paper. A prototype valve will be available in 1995–96 for incorporation into an integrated vacuum system test loop.