Zero-knowledge (ZK) hardware is computing equipment built or optimised to produce zero-knowledge proofs. Checking such a proof is quick, but creating one is slow and costly, because it involves huge numbers of large-number calculations. That cost limits systems such as ZK-rollups and zkML. Hardware approaches range from running provers on graphics cards (GPUs), to reprogrammable chips (FPGAs), to custom chips, the same kind of ASIC used in bitcoin mining. Several start-ups and research groups work in the field, and some networks pay independent provers to run such equipment. The market is young, proof systems keep changing and a chip designed for one may not suit the next, which makes custom hardware a risky bet.